And what has this to do with social skills which were until now the focus of this discussion?
*sigh*
Because social skills are also another type of mental skill. Charisma and Social Skills represent fitness within and control over a social environment. As such, the actions of the character depend intimately on the choices of the player.
Consider the iconic case of being caught by the guards sneaking in the Baron's house, and deciding to bluff your way past the guards.
Scenario #1:
Player: "I'm going to bluff the guards. I have a +9 in the bluff skill.... 17 with modifiers that's a 26."
DM: "That's a really good bluff, the guards decide to let you go."
That's 'role-playing' in the sense that a cRPG is 'role-playing'. But it's missing well pretty much all the things that make role playing interesting. Even more importantly, we don't know what the guards believe now. We don't really understand from this where the story goes. We could have the DM decide what the guards believe, but that would be indirectly roleplaying the PC since the DM has decided what the PC said. We could have the DM and the PC negotiate over what the PC said, fortune at the beginning, but this still relies on the DM deciding what works and probably involves (as the next example will demonstrate) passing meta-game information to the player. True roleplaying, in the sense we'd want to make social skills match the decision to attack something with a sword, open the door on the left, or climb the north wall involves greater specificity over the player's actual choice.
Scenario #2:
Player: "I'm going to bluff the guards. I have a +9 in the bluff skill."
DM: "But what do you say."
Player: "I'm gong to imply that I'm the Baron's new mistress, and I was sneaking around to be discrete."
DM: Ok... roll the dice.
Player: 17... and with modifiers a 26.
With a choice, the mental judgment call from the player, however comes a problem. We can assume that the character projects absolute sincerity even the player lacks this skill, but the particular lie may or may not be suited to the situation - just as charging into a particular square in combat may or may not be suited to the situation.
Consider the information that the player might not have, which the DM at the time may have already written into the scenario:
a) The Baron is a renowned lecher, and young women come and go all the time. This is a highly believable bluff that probably shouldn't require much skill to make, and which is unlikely to cause much comment. Is the DM wrong to make the DC low?
b) The Baron is a 80 year old man renowned for devotion to his wife. So even if this unlikely bluff succeeds despite high assigned DC ("I'd sooner believe it is going to snow at the midsummer festival!"), the PC's bluff may create a scandal that causes the lie to unravel spectacularly. Is the DM wrong to make the DC high?
c) The Baron is known by the guards to only be attracted to men/young boys/goblins/merfolk/etc.
d) The current mistress of the Baron is well known to the guards, who are used to arranging all such trysts.
e) The Baron has just instructed the guards to murder his new mistress, whom they've never seen before, and a case of mistaken identity has just occurred.
Each of these cases requires the DM to make some judgment about the suitability of the lie and its consequences (whether it is believed or not). It's clear that different choices by the player influence the outcome of the social challenge. Now consider that the player may have learned IC one of the above facts (which ever is true), and at the time he's asked, "What do you say?", he recalls the fact and realizes its importance in the situation so that it informs the choice he makes. A less perceptive player, might not recall that information or its suitability or even attempted to research such facts before deciding to go house breaking - despite the fact that the character ostensibly is intelligent and we'd expect them to behave differently. What are we to do though, make the choices for the player, depriving them of agency?
In this way, we can show that social skills are not of the same category as say the ability of the character to jump. A physical skill like 'jump' says the character can jump, but not when to jump or over what obstacle. Nonetheless the chance of success isn't really dependent on a player choice. A social skill however says the character can bluff, but the chance of success is still dependent on a player choice and understanding.
Now, if the DM knows nothing about the Baron, and he decides things on the fly, then the DM is railroading in some fashion or another, even if he's ruling favorably - ei, DM now jots down that the Baron is a lecher to make the bluff work.