How accurate are the calorie calculators on MFP and Map My Fitness?

I have Map My Fitness synced to my MFP app but I'm wondering how accurate the count is. For instance, I rode my bike 2 miles today and it said I burned 162 calories. But yesterday I walked a mile and was told I burned 183. What?! I know for a fact I sweat more and work harder on my bike. Yes it is a cruiser setting but I still have to work harder than walking. I do have my weight and height recorded. Anyone have advice?