Not every day someone gets this badge ... If someone already has it earlier and buys the first one, no one gets it on a given day
I was also assuming the same thing. The achievement is to be "the first one to buy the card", NOT "the first one to buy the card, that does not already have the achievement".
As a percentage is used instead of a count we cannot determine that 4900 individual users that have earned this achievement. Plus the users themselves will only know the first time, if they are the first one to buy a card.