The dome is usually higher than an oem piston crown as opposed to us making our own on a lathe and dropping the cylinder over it.
One can weld a popup to a crown and turn it down as well, but a manufacturer has the ability to have their slugs cast and machined any way they want them.
BTW, the reason the BB 660 kits free port so badly is that they have shorter skirts than oem. That’s done to insure they fit the case they are put on. The factory 54mm slug’s skirts dip below the case pad at BDC. Usually there is not enough side clearance to allow the 56mm slug to drop down into the case, so the AM responded by removing the skirt. That’s why the Freeport so badly, it’s not the jug design, it’s the intentionally short skirt.