I've noticed some games run with less stuttering when I apply a 60fps lock, when already using (adaptive)v-sync on a 60Hz screen. Why is this the case if the frames are paced directly with my screen's refresh?
It depends on the game engine behind the game.
If you just run a v-sync, the engine probably don't know this. It will generate 60+ frames, but not all of them were directed to your display. Which means some of them just bounce.
If you define it in the game itself, the engine knows about it and can react. If the engine produces a world or a scene and probably can create 70 frames per second, it just stops at 60 frames. Now the engine can do something else for the 10 frames. It can probably load some textures which will be needed soon (in the next seconds). It can probably generate the models left and right from your viewport, which means they are just instantly ready if you spontaneously take a look to the left or right. Or it just can calm down and give your graphic card 10 frames time to rest and to cool (or not to heat up). Which will produce better performance in the following seconds.
Hopefully this will answer your question otherwise, just ask in the comment.
Your comment about locking FPS in RTSS, disabling in-game v-sync and using adaptive v-sync in control panel changes the question quite considerably. In this scenario, game can have less stuttering (and lag) because you are running it without v-sync, FPS will never go above your monitor refresh rate (here 60), so adaptive v-sync in drivers will never be utilized.
But this would mean you would see tearing because outputting frames from game and refresh rate of the monitor are not synced (even if it's 60FPS@60Hz). Do you see any tearing?