Melissa, that's fantastic! You really make a lot of progress; you must feel great!
Mami, as I think you already suspected, the machine tallies aren't accurate. See the thread
http://www.3fatchicks.com/forum/showthread.php?t=88023 in the Exercise forum. According to the article, they can be off by anywhere from 10-30%. There are just so many factors that the machines don't consider, such as room and body temperature and body fat % (muscle burns more calories than fat), that they are an estimate at best. But the calories are a good way to keep track of the intensity of your workout. I always track my calories on the elliptical because I control the revolutions per minute (unlike the treadmill, where I just have to keep up with it). I aim for a certain number of calories per workout and if I hit my number, I know I'm working out at my usual intensity. And it's also a good ball park estimate. You may not have burned exactly 735 calories, but you definitely used a lot of energy. Even if the machine was off by 30%, which according to the article in the thread, is extreme, you still burned over 500 calories--way to go!