That can happen..
if you have sent them a lot of downloaders and registrations, they might have sent out a mailer to get them to deposit..
these 20 new depositers are not necessarilly from this month only.
I notice this at a number of casinos who have a good conversions department that registrations from previous months suddenly turn into depositors. Ask your affiliate manager if they did some sort of mailing. Most of the time that would be the reason.