I am taking as a given that my chain is worn and that the chain checker (in my case, a new Park Tool CC-3.2) is either accurately gauging the chain or, as suggested above, overestimating its stretch. I had not considered the possibility that the chain checker was underestimating the chain stretch, however unlikely that possibility may be. I will check this out tomorrow using a measuring tape and a longer segment of chain.

The fact that the chain is worn, however, is not the surprising thing to me. The chain is the "consumable" part, after all and is meant to be replaced regularly. Or at least more regularly than the other components of the drive train. However quickly I reached this state, be it X months or Y miles, to me does not seem to matter. What I did not expect was for the cassette to also wear out in the same amount of time/miles. In my mind, I thought that even though the chain and cassette both operate in the same environment (comparable levels of dirt, moisture, etc.), the cassette is more durable and the chain wears out more quickly, which can then wear on the cassette teeth to match its ever-growing stretch. Implicitly, I am assuming that the primary wear mechanism for the casette is a worn chain. If I am wrong, please correct me.

As mentioned by "alcjphil," if I am constantly using only a couple of the cogs on the cassette, these cogs will wear more quickly. I agree, and that seems to be what i have observed, with the skipping happening more frequently (but note exclusively) on these cogs. Don't think that I never change gears or anything like that; I definitely do. These extra-worn cogs are the ones in the middle of rhe cassette, which to me seems reasonable to spend more time in the middle than on the extreme ends.