Calories burned for exercise

Why do the monitor things you get on exercise machines that tell you the calories burned, always tell you that you burned way more than all the sites online that will tell you calorie burns for exercise? because when I go on my stationary bike for 35 minutes at about 10mph, it tells me I burned over 300 calories! I know I didn't burn anywhere near that because it was pretty light effort and also all other calorie things on the internet say much lower numbers.

Those machines, as well as these websites, are set with calculations that give approximations.  The only way to tell how much YOUR BODY is actually burning is to hook you up to a heart rate monitor and a breathing apparatus and a bunch of other stuff.  I spoke to my personal trainer about this same question, because the difference was very dramatic, and he said that the Calorie Count estimates for how much I am burning are probably the most correct - so I would go with that for reference.