One is a mechanic and the other isn't? We aren't just talking about results here - the difference is the tool, not the output, and different games use different tools, which include systems, mechanics, and advice, to accomplish different design goals.
Here's a brief episode of RPG play:
My PC arrived at the dungeon entrance, and went down the stairs. There were two Orc guards waiting at the bottom, and they attacked me! They drove me off, and I had to run back to the village to look for help. But all that happened was that the villagers mocked me for my cowardice and failure.
Here's one way that could happen, in play; the system is classic D&D:
The GM tells the player, You've arrived at the dungeon entrance. There are stairs going down into the ground.
The player says, "I go down the stairs."
The GM checks their notes, and sees it mentions two Orc guards. The GM rolls for a reaction - the guards are hostile - and calls for surprise rolls - the PC is surprised. The GM rolls attacks for the Orcs, they result in hp loss to the PC. When the surprise is over, the player wins the initiative, declares the retreat, and survives the rear attacks from the Orcs. The GM, noting that the Orcs are guards and not harriers, and also that it's still daylight outside, decides that the Orcs don't pursue.
The player declares that their PC goes back to the village, and the GM accepts that. The player then says that their PC looks for help; the GM rolls a reaction for the villagers, and it is low, and so narrates that as jeering instead.
A variant on the above: the GM's notes don't list anyone at the bottom of the entrance stairs, but do say that, when a PC arrives at the dungeon, the GM should roll for a wandering monster (because this is the well-travelled interface between the outside world and the underworld). And so the Orcs are the result of a wandering monster roll, and the GM narrates them as being on lookout.
Another variant, on a different part of the above (and so quite compatible with the above variant): when the PC flees, rather than making a decision the GM rolls a morale check, with a penalty for the daylight, to see whether or not the Orcs pursue.
Here's a more different variation, which replaces some classic D&D procedures and mechanics with more 2nd ed A&D-ish ones:
The GM tells the player, You've arrived at the dungeon entrance. There are stairs going down into the ground. Do you go down?
The player says, "I go down the stairs."
The GM checks their notes, and sees it mentions two Orc guards who attack intruders. The GM calls for surprise rolls - the PC is surprised. The GM rolls attacks for the Orcs, they result in hp loss to the PC. When the surprise is over, the player wins the initiative, declares the retreat, and survives the rear attacks from the Orcs. The GM, not wanting a TPK so early in the session, decides that the Orcs don't pursue.
The player declares that their PC goes back to the village, and the GM accepts that. The player then says that their PC looks for help; but the GM decides that a better fit with what has happened is that the villagers jeer the PC, and so that's what happens.
And here's another variant, that assumes Burning Wheel as the game being played:
The player has built a PC with a mostly Outcast background, which includes a difficult relationship with a former mentor. It's agreed between player and GM that the difficulty is that the mentor is held prisoner in a dungeon. And the player writes, as one of their Beliefs, "I'll free my mentor from the dungeon." And has an Instinct, "Always Assess when entering a dangerous place."
The GM starts actual play by telling the player, You've arrived at the dungeon entrance. There are stairs going down into the ground.
The player says, "I go down the stairs." And adds, "This seems pretty dangerous - I Assess!" The GM asks, "What are you worried about?" "Guards," answers the player. The GM decides that there's a reasonable chance there are guards here, but probably not stealthy ones, and so set the difficulty at Ob 2, The player fails the roll, and so the GM has to narrate the consequence. The GM notes that the player has Orc-wise, and so decides that there is a pair of Orc guards, who attack the PC.
The fight is resolved using Fight! The PC successfully Withdraws, up the stairs, and then the player declares that the PC flees. And declares, "Everyone knows that Orc guards don't pursue intruders in the daylight - they're not harriers!" The GM calls for a roll on Orc-wise, at an appropriate obstacle; the player succeeds, and the PC's knowledge of Orcs is accurate: there is no pursuit.
The player declares that their PC goes back to the nearest village, and the GM accepts that - there's nothing at stake. The player then says that their PC looks for help; the GM calls for a Circles test. The PC has no Village lifepaths, and so the only help they can hope to find is outcasts and vagabonds like them. The Circles test fails, and the GM narrates the failure: the villagers want nothing to do with this outsider, and certainly offer no help. Rather, they mock and jeer.
Same "story"; pretty different play experiences! And those differences result from differences of rules, mechanics, procedures, principles, etc.
In an old thread on these boards, I described two different ways to create four piles of cards, with alternating red and black in order from A to K. One is to open the box of cards, sort all the cards, and stack them like that. The other is to successfully play out a round of solitaire. Same result/output - but pretty different experiences, and only one is a game!
It's obvious in the case of the cards. It also puzzles me that, when it comes to RPGing, so much discussion assumes that all that matters is
the content of the fiction, as opposed to
the game whereby that fiction is created.
