Yes, the discrepancy is dry quarts vs. liquid quarts, and the sellers using liquid quarts are probably hoping no one notices that by actual volume a dry quart is 16.4% larger than a liquid quart. If I apply that correction to the sellers claiming 4 cu. ft. / 120 quarts, I come out with 103 dry quarts. Both brands I am looking at online state 4 cu. ft., so then it’s just a matter of price and possibly density / weight, where the desired lighter perlite should show up with a smaller shipping weight (assuming a significant difference in density.)
Silly me — I should have spotted the dry vs. liquid issue. Well, it was early...
This would show up in recipes too: If a recipe calls for 3 cups of flour and 1 cup of milk, should one use a dry measure cup for the flour and a liquid measure cup for the milk? Or are all the measuring cups likely to be used made for liquid measure, and that is taken into account in the recipe?
Even more fun (HA!) is the example of trash cans and trash bags. They are rated in gallons, even though I would hope no one is setting out, say, 42 or 55 gallons of liquid waste of some sort(!) for pickup.* The trash container our new trash pickup company provides us is even bigger. (I should try measuring it.) One of that big size is faster than multiple containers for automated pickup, I suppose.
*Good heavens, if we had 100(?) gallons of liquid waste set out, I imagine we’d either see it left roadside, or we’d get a BIG surcharge for some sort of other pickup. (Send out a septic tank service truck?)
1 cup, stainless steel, flat top -- 1 cup, pyrex with pouring spout. Exact same. SS cup also says 8 oz on the bottom, not 8 fl oz. Dry weights can't be measured in a container because one substance will be heavier than the next. Probably all figured into recipes as you say.
Meanwhile, there's a reason pro chefs go by weight and not volume for non-liquids. Even chopped veggies that will fit in a cup might weigh different due to variety, juice level, chopped size. I don't think they have dry measuring cups and for that matter, might even weigh liquids, not sure on that one but I'm thinking by volume, 1 gallon of the raw milk I can get from the Amish weighs different than one gallon of 2% from the grocery store.
The English came up with screwy systems of measurements and we inherited it. How many hands is that horse? Depends on the size of your hand. Dry quart is something I've never heard of until this morning.
There's a reason older women don't measure anything. Pinch, dash, dollop, good amount, good sized roast etc