Isn't 1080p 25fps?
That depends on where the video processing is being done, and where in the
video signal path you make the measurement. The original source material
may have a lower frame rate (1080p24, 1080p25, or 1080p30), but the video
signal to the iMac's LCD panel or an ordinary LCD monitor must be refreshed at 60Hz.
http://en.wikipedia.org/wiki/1080p
LK