Compare to the other symbols in that set: a square refers to the drying process, with the contents being how to dry it. A circle instead of a square means no drying process, so it shouldn't get wet in the first place - hence dry-cleaning.

At least that's how I see it, it could also just have been the last obvious symbol remaining after square/triangle were taken.

Since dry-cleaning isn't "dry" (it still uses liquids, just not water) it should be a simple drop of water symbol with the universal "NO" bar across it like this: https://www.istockphoto.com/vector/blue-drop-or-droplet-of-w...

That makes "do not dry clean" difficult, since it'll effectively be a double negative.

(I do think the circle is difficult, but that's probably because I don't typically wear clothes that one would think about dry cleaning)

Then you'd just have a drop, but true, there are three states - dry clean ok, dry clean bad, dry clean mandatory (where the first is do either, but that could be indicated with no symbol at all).

How do you tell a drop of water apart from a drop non-water liquid?