Accuracy of cardio machine calorie estimates?

LittleRedBoots

I did a workout this evening on a stationary bike. I entered my weight, height ,age and gender and it estimated my calories.
I am 62kg, 19 years old, 174cm and female. I biked for 30 minutes working reasonably hard.
I kept revs between 90-100 and it said I went ~17km. I had it on a medium gear setting.
After my workout it claimed I had burnt 653 calories.
This seems like I lot to me considering my 20 minute run burns about 420 according to MFP.
Can anyone shed any light on this?