May be more to the story....they are not allowed to sell partial boxes of the tic/flea stuff...so some times it works out to get the vet tech to dispense it (by law) and get only what you need.
I had a Large breed pup that needed only 2 doses in one weight category and then the rest of the summer he was going on to a larger dose. So they recommended coming in for the two instead of buying a whole box.
Possibly something similar with that scenario....