First of all, welcome to PG. Hope you enjoy your stay here.
It sounds like they are talking about two different things. Garth has sold over 100,000,000 records, not 97 platinum records. As far as Elvis, I'm not sure if the 50 is referring to the number of platinum records he has or that he has sold 50,000,000 records (although I think he has sold much more than that).