Can't get over 60 fps on mac

I play Minecraft on two computers, a 2.66 GHz 2010 Mac Mini and a 1.8 GHz 2012 Macbook Air. The Mini, being older and less powerful, can usually give me a good 40 fps in singleplayer when on fast graphics and 4-chunk render. It maxes out at 60 fps when I stare into nothing but air blocks. I thought it was a bit strange that it seemed to cap at 60 even when I had set "Max FPS" to "Unlimited," but I thought hey, it's a 4-year-old computer, maybe that's just how it runs. When I heard that 14w30b came out, I immediately watched SethBling's overview, in which he was able to go from 40 fps to 215 just by looking down. He did that on something like 12-chunk render, so I thought surely I could do the same on 8-chunk, since that usually nets me a good 40 fps on my Air. I tried the same thing on the laptop (after updating to 14w30c), and instead of getting over 215 fps, I got 60. Then I created a customized superflat world with nothing but air blocks and villages and pointed my cursor into nothingness. Still 60 fps. I checked the slider, it still said "Unlimited," I checked for other apps running but I had quit them all.

Not that I'm complaining, because 60 fps is still a very good framerate, I'm just wondering why it caps at that when the slider goes past it. The slider caps the framerate just fine at anything under 70, so I don't see why it wouldn't do it for 70 and over.