There is a difference between losing weight at first vs. losing/maintaining long term.
The equation of calories in vs calories burned is universally true. So, at first, your body is used to a daily intake of a certain number of calories in relation to the amount of 'work' you do. You reach a equilibrium. (Assuming you are not constantly gaining.)
When you suddenly add in biking (or any extra 'work') you are now burning more calories and that changes your balance. As long as you don't up the calories, you will lose weight just by adding in exercise without any diet changes. When folks start to add exercise I think they generally make--at least some--small calorie reduction.
But, you will reach a new equilibrium when you once again balance the number of calories in vs you new level of calories burned. I think that is why weight loss tends to be so dramatic at first then tapers off/plateaus.