I noticed the same thing with 1 bottle.
Here is my theory: the UC that is watery might have been exposed to too much heat at some point during processing or possibly shipping.
Here is why I say that. I have bought like 4 bottles of UC. I love the stuff. Even the watery consistency still works the same. Only 1 bottle was thin and watery. You can tell just by shaking it at the store before you buy it. I noticed with the watery thin batch, the bottles (unopened) seemed to be "bloated" for lack of a better term.
So, again just a guess...product is placed in the bottle and sealed at the factory. It gets exposed to too much heat at some point, like a pallet out in the sun all day or something. The heat affects the liquid, thinning it out and expanding/bloating the bottle.
So before you buy your next bottle of UC, shake it first and you'll know right away if it feels right. If it feels thin, turn the bottle sideways and see if it is bloated. In the end, it doesn't matter because the product works the same, you just gotta pay attention a bit more so it doesn't run off the pad when applying it. Again, I love the stuff. It works awesome and is only $8 OTC!
I would love to hear the Meg's chemists take on my theory, or anyone else's take for that matter.
Justin