It's complicated, but basically the game makes use of two back buffers (this is a requirement for flip model which the game makes use of). On top of this I believe in some scenarios there might also be a third front buffer.
My current hypothesis is that this depends on what presentation mode the OS is able to grant the game, but I need more info from Kaldaien to say for sure.
Basically flip models requires two back buffers, but as far as I understand it when engaged in the best optimization mode where the DWM of Windows steps back there wouldn't necessarily be a third front buffer -- a direct scanout would occur straight from one of the two back buffers of the game.
This would, I guess, result in the noticeable double buffered v-sync behavior where frame rate drops from (Refresh Rate / 1) to (Refresh Rate / 2).
However, there is also composed situations to consider... If the DWM is involved, and a direct scanout cannot be achieved from the back buffers, a third front buffer would be involved where the contents of one of the back buffers would be composed along with contents from other buffers on the system.
That would, I once more guess, set up a scenario where the FPS can render unrestricted in the two back buffers without dropping down to (Refresh Rate / 2) when the FPS drops below (Refresh Rate).
So what we're seeing here might simply be the result of different presentation models engaged in Windows -- the guy who experienced the double buffered v-sync behavior might have Windows engage a more optimized presentation model than those that do not experience that behavior.
Aren't games complicated?! :D
This is just my current hypothesis that would explain everything -- I haven't actually confirmed it myself yet. I've also pinged Kaldaien on the Special K Discord server to check with him if this sounds like a reasonable assumption/explanation.