They are only allowing those with either an AP or the AP voucher. There are numbers on the back of the voucher that must be given to book. They can tell if that voucher number has been sold (just not to who unless paid for by a credit card I would imagine, even though that doesn't mean that is who it is for) If you have the voucher, they will more than likely ask for the proof at check in. If it is already activated by check in, they will have all your info.
They not only can tell you how many times you have gone, they can also tell you the names of everyone that has ever stayed in your room. When ever I call, they always ask me about 2 or 3 other names.... friends of my daughter's, or my aunt's name. They
do and can keep track...they are like the FBI when it comes to your profile