Outdoor Cycling Calorie Estimates

Over the past few weeks the estimated calorie expenditure for the same outdoor cycling workout done 3 times (12 laps / 16.5 miles around a reservoir) have yielded in this order 450, 633 and 263 active calories at the same pace 17..6 mph and average heart rate 109 to 114. I have not experienced this issue riding up and down paved trails and roads or around the reservoir last spring to fall or the previous year spring to fall. I have an Apple 3 watch. The estimated active calorie expenditure for my 5 mile runs has remained consistent.

Any ideas/suggestions?