I've had two very different experiences with varnish in the last few weeks.

Experience #1: I picked up a can of Z-Spar from a boat shop that was conveniently located on my way home from work... not a supply shop, just a boat shop. It seemed very viscous and quite color-dense as I applied it. I did not thin it (which I know now was probably a mistake).
Results: WEIRD. it took forever to dry. The application instructions say that it should be tack-free in two hours, but it was still somewhat tacky 24 hours later, and could easily and deeply be scratched with a fingernail even days later. This was in a detached garage at 50 to 60 degrees.
Since I didn't thin it, I had some drips and sags that I wanted to sand off. Since I had some other things going on, I didn't attempt any sanding for at least a couple of weeks. When I got around to sanding, I noticed a few (10 or so) small dots and wavy lines that didn't sand -- the varnish balled up and gummed up the sandpaper. In these spots, you could scrape the varnish off with a fingernail. Like I said, WEIRD.

After all this, I just accidentally happened to notice that the manufacture date on the can was 1996.

Experience #2: I picked up a second can (2000 this time) from West Marine, and thinned it a bit with T-10. WOW! what a difference. It goes on much smoother, isn't nearly as dark, doesn't sag, and dries and hardens rapidly. I've now got one boat under an A-frame plastic sheet with a space heater, but even the unheated boat's varnish dries rapidly, and at cooler temperatures than before.

So my question is, wth happened? can "old" varnish undergo chemical changes? lose its volatiles? what's going on there?
Or is what I saw purely the effect of thinning it?