is that it was originally developed to spray down rockets to displace and remove water, to insure against the thing having to haul ice aloft as it struggled to get off the pad.

That's where the WD in its name comes from. "Water Displacer - formula #40". It was specifically designed to grab water and drag it off of metallic surfaces, leaving the metallic surface with a thin lubricating coat, which is designed to shed water.

The fact that the product can be used in a zillion ways in homes, shops and industry is a fortuitous happenstance.

So, the fact that used WD-40 contains a lot of entrained water is not only not a surprise, it is anticipated.