My understanding is that typical 20lb tanks and sizes that are filled and transported commercially for resale at gas stations and what not can't be filled to max.

While you or I can take the same tank and have it filled to the max level. I have a few 20s and a single 100lb tank I use for a water heater. The 20s almost always end up at 4.5 gallons and the 100 is usually around 23.6 gallons or so when I get them refilled.