Not All Terminal S's are Plurals (or Possessives)

It is the nature of my job that I'm beset with lots of different systems with acronyms, many of which happen to have S as their final letter, usually standing for "System." To invent a non-real one, assume something called the "Cattle Amplification Teleportation System," known as CATS. For reasons I do not understand, my co-workers (and I suspect people generally) have a tendency to assume that the name of every one of these terminal-S abbreviations is either a possessive or a plural. In this example, I can find that particular non-system spelled both CAT's and CATs in the same document. For some reason this bugs me, and I find myself correcting their spelling. When I try to gently point it out to them, they look at me funny. As far as I can tell, they can't tell the difference; it's invisible to them.

I'm not perfect. I make its/it's errors all the time. But why do so many people assume that if the last letter of any word is an S, it must have either an apostrophe before it (the grocer's apostrophe) or that it must be making a plural of the letters before it?