The bike's estimate of calories burned is probably closer to accurate. 900 sounds way too high for a 40 minute workout. Are you sure you used the stationary bike entry in the tracker, and not the road bike (which would be much different)? Your speed also sounds pretty fast. Are you sure that number is accurate?
As far as calories burned through strength training, that is all based on very rough estimates. So I wouldn't get too caught up in the numbers. If the exercise takes longer, you're probably using good form and it will be more difficult, vs. rushing through it and probably not burning as many calories (and also not getting as much benefit.)
Hope that helps,
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford