i know that the harris-benedict formula that spark uses has this issue and my guess is that anything that doesn't use your actual body composition will as well. but the higher your weight, the more inaccurate the formula is. basically the formula is programmed for you to have a higher amount of muscle at the higher weight than you likely do, and thus the formula assumes that you're burning a slightly higher rate of calories than you really are.
if your tdee is taking into account your exercise, then that's the difference between your bmr and tdee because your bmr would only be taking into account your daily activities, not any exercise.
if you're 5'7" and female and 1800-1900 works for you, stick to it for now. you may have a slightly higher rate of loss to start out, but 3 lbs a week isn't unreasonable if you weigh 285 lbs to start. as you lose your loss should slow down as well. eventually you'll likely need to dip a little below that to lose, but your end maintenance calories should be somewhere around there, so it's not a bad place to work from. if you need more calories and can still lose weight, that's fine too. but you're likely in a bit of a play it by ear place. look to your workouts and how you feel doing them. if you're fueling them, you don't need to eat more. if you're feeling weak, add in a few hundred calories and see how that suits you. in addition to your workouts, look at the results on the scale. if you're losing and satisfied in your workouts, that's the place where you want to be.
-google first. ask questions later.