You're right. They're not thread sizes in the traditional sense. Think of them as thread styles instead. Unlike a 510, some of them, like the 808, use a male thread on the battery side and a female thread on the carto side.
The 510 is the most common. To confuse you even more, some other styles fit on a 510, for example a 306, (and maybe a 901) atomizer/carto fits a 510 threaded battery connector. But their drip tips won't fit a 510 atomizer. Many people prefer a 306 atomizer for dripping. It's built differently than a 510 atty and makes a hotter vapor. But it does fit a 510 battery connection, hence 306 refers as much to a style of atomizer as a size of thread.
Also, a 510 carto/atty fits an ego thread. But a carto/atty made for an ego won't fit a 510 without an adapter.
Fortunately, unless you're particularly fond of 808 cartos, and some people are, you can just stick to 510 threading because they make 510/* adapters and nearly every type of carto/atty/tank made is made in a 510 version.
Best thing to do is go to some vendors sites and look at the different attys and cartos. A good one is Ikenvape.com because they have a lot of atomizer styles and good, clear photos where you can see the differences in construction.