Typically these matches are only for publicly available rates.

Meaning if you saved 10% for being a hotels.com member, which anyone can be for free, but that 10% sent you over it doesn't count. Only the publicly available rates that you could book without registering counts.

Was your booked rate considered publicly available? They can't tell from the reservation you have that it was. You need to PDF the page where you found it, in case the price changed later, your Booking confirmation won't tell them this either