One's taking 2.67 milliseconds, and the other one is taking 3.61 milliseconds. FPS can give misleading measurements. Or rather, FPS makes the measurements seem more dramatic than they already are.
Losing a single frame from 20 FPS to 19 FPS is a much bigger deal than losing a single frame from 100 FPS to 99 FPS.
That said, you're still measuring some difference (one is taking 1 millisecond more per frame).
Your two screenshots look different though. One looks like the blocks are casting shadows (or is that AO?) with some gradient shading and rounded edges, and the other is really sharp with flat shading. They look significantly different, at least to my beginner's eye.
Are you positive you didn't accidentally map your rotation keys also to toggling on/off special effects?