1000 calorie workout, never reaching anywhere near 1000?

Hey guys I've done my third 1000 calorie workout now, and each time I've never made it to 1000 actually I'm really under it, my calorie burn is normally around the higher version, but every time I do these I'm barely making 600--- it's around the 500 mark most the time, anyone else have this issue? I'm pushing all the way through it but cannot seem to get the calorie burn!