Calorie Burn

bikrchk

So I got the Garmin ViVi Fit fitness watch. It's meant to be worn 24 hours a day and connects wirelessly to their heart rate strap for workouts. I like it, but I've noticed weirdness from day to day regards calorie burn. I ride a stationary bike 5 days per week for an hour a day. Hill intervals. I run it on level 16 or 17, (it has 30 levels). Most days it calculates my burn at 550ish calories, but some days it says 430ish. Heart rate is similar, length of workout is similar, weight is no more than a pound or 2 different between days. What is going on?