Cyclemeter calorie count

How does cyclemeter count calories burned? When I ride my commuter bike at 11.97 mph for the 5.6 miles it takes to get to work, it tells me I burned about 400 calories.

Today I did the same run on my fixie and I averaged 10.98 mph and it says I burned 475. Now it was more work (being a fixed gear) but there's no way for cyclemeter to know that.