There’s this consequence of manufacturing where when they put, like, 4 ultras and 2 secrets in a box, if the 12 packs on the right (assuming a 24 pack box) have a secret rare in spot 1, and an ultra rare in spots 4 and 9, then the left side will also have a secret rare in spot 1, an ultra rare in spot 4, and an ultra rare in spot 9. Apparently this is not true for all manufacturers, and it’s more prevalent in some regions than others.

But basically, if you buy one pack at a time, you could be buying from the “other half” of a box where someone took all the packs from the left side, then the most valuable packs from the right side, selling you something unlikely to hold a high rarity card (though I think you could still get a starlight or QC rare since they do not try to get them in a certain proportion per box).