The 90 target boxes are in the big box retail stores, like Walmart. That must be the way they want them. The gun clubs buy the 130 target boxes, as ctreay said. That gives you a full round for 5 shooters plus a "let's see one" and a couple of broken / failure to fire targets extra. Bill Malcolm
Gun club cases are packed 135 because a skeet or trap squad is 5 shooters x 25 targets per round = 125, plus 10 extra to account for breakage and show targets. That was the rationale when the target companies first started 135 packaging in the 1930's.
I think guys like Walmart that sell 90 instead of the 135 do it because they can have a cheaper prices than thier competition on the clays even though there are less in the box. Most guys that shoot for fun don't look at the number of clays only the price.
It's about perception and value pricing. Unless you're shooting trap or skeet where 25 shots are the norm for a round not having enough targets for five rounds is a none issue. The walmart or other big box customer is the guy with a hand thrower or a Trius hand cocked just messing around behind the barn or in the field with hisw friends. Target case size may have more to do with case count on a pallet than anything else given the fact its retail sales per square foot that drives the mass merchant.