It varies - what's so surprising about that? If you think the barrel-banded thin glass things typical at many homebrew shops have much relation to the thick, smooth-walled acid carboys, you've got another think coming. Even among acid carboys, different manufacturers vary somewhat - and whatever the case may be with how few places are still making them, they can last a long time, and have been made by many places.
If you want to graduate the thing, select your precision measuring device of choice and measure in precise amounts of water, then mark the outside with a sharpie.
I have one mark, on both sides of my primary carboy, which simply tells me where one of my secondaries, full, comes to. That lets me know what I'm shooting for. The secondaries in question typically are about 5.25 gallons up into the narrow part of the neck, which is where I like to secondary. If I get the primary a bit over the line to account for trub, I get a full secondary. It's not a rigid rule, as I do full boils, so my primary volume measurement is what's scratched into my kettle spoon - I don't normally add water.