I looked online and it is 700 calories for a 75 minute class which seems pretty good to me.
You have at least a 5 minute warm up, if not 10 minutes, then at least a 10 minute cool down. That leaves about 60 minutes for the bulk of the 700 calorie burn.
Not a figure that is reliable. Some people will not put as much effort into the session as others, so the calorie expenditure will not be the same for everyone.
A figure like that as an across the board number has come from a person who needs to qualify their statement as can be very deceiving. As I mentioned above, calorie expenditure is not really that important as it is fairly meaningless overall.
If it works for you, great, but you asked the question in the first place. Just don't expect 700 calorie burn per session realistically.