External display : why 59Hz instead of 60Hz on Macbook Pro 15" mid-2012 @ 2560 x 1440 via DisplayPort I connected an LG 27UD88-W to my 2012 15" Macbook Pro via DisplayPort.
However, it does not seem to connect at the expected 60Hz for the 2560x1440 resolution, but instead it shows 59Hz.

One could argue "oh well, you won't notice that 1Hz" but that is not the point.
I like to understand things.

Does anyone have an explanation for this?
You can pretty much always attribute this one to a rounding error in the way the information is displayed to you. The actual refresh rate is somewhere very near 59.97Hz, leading whatever function they've run the number through for parsing to only display the whole number digits, and not perform any actual rounding.

This does lead to some interesting effects though. Some applications will do this and end up with a V-Sync rate that's not actually synced (They fake it and do framerate lock instead, leading to 59fps when you actually need 59.97fps exactly.)