They were numbered in the order that they were “discovered” (or theorized, in the case of population III). Similar to how exoplanets around a particular star are designated “a”, “b”, “c”, etc. in order of discovery, so they usually end up not in any particular order in terms of orbital radii or size. Sometimes letters end up skipped when a letter is assigned and then it’s discovered to be a false positive.

So there’s logic to it, just not the logic one might prefer.