Far cry 3 borderless windowed mode, why is it so great?

so i have a gtx 780, and a core i7. i can max fc3 at 1080p and it feels like a solid 40fps while full screened. I switched to borderless windowed mode and now suddenly it shoots up to like 100+ fps without vysnc, and a solid 60 all the time with vysnc.

i mean, wow, what a huge difference. anyone know why this would be from a technical standpoint?

Usually the other way around but it depends on a ton of factors....the game coding being the main one.

@REforever101: My problem is I cant get the darn game to run in 16:9 ratio to match my monitor. I'm running in 1080p but everything looks squished

Just tried it, no difference for me. You might be getting a bit less performance than you should. I have 290x, 4770K, and it almost never goes below 60.