One is about the physical reality around the character, the other is about what the character believes to be true.
The physical reality around the character is the purview of the DM. What the character believes to be true is entirely up to the player.
The player cannot simply decide that they're sneaky and no NPCs are aware of their presence, that requires a check. The player can simply decide if they believe an NPC or not, regardless of any check. Putting a check in there gives the statement an objective truth value that is impossible for anyone without telepathy to have. It's not up to the DM what the PC believes, it's up to the player.
One points externally, the other internally. That's the difference.