[pwg]

If a given retailer gets the same number off your card each time you do contactless, then that retailer /could/ track you via that number.

If all retailers get the same number, then they can each track you, and correlate your purchases between themselves.

Note, there just needs to be /some/ constant number from whatever comes through via contactless, the number does not have to be the magic numbers that post the sale to the card.