It's not just a clever name, these things really are perfect. They fit perfectly in two senses: the cards fit perfectly inside the boxes (occasionally you will want to go a little fewer than the listed count to make it easier to get them in/out, and occasionally you can go a card or two over), but the boxes also are great for fitting sleeved cards into wargame boxes that are already short on space.
I have several titles that would struggle to close with ziplocs full of cards that are fine with a 3x3 grid of 15- or 22- count perfect fit deckboxes.
Very easy to recommend.