OK, just in case it helps anyone else, I figured out in the end what must've happened before. There's a section on the eBay front screen itself called "New For You" (see screenshot attached.) I'm unsure if this is a new thing or just something I've never used before. But anyway there was a reminder underneath that heading that I had two items to send. And since I needed to buy postage labels for them this seemed at the time a good a way as any to get them.
Having gone back and investigated further though, this front-page link takes you directly to the "Get labels in bulk" screen. This is not something I'd encountered previously and works completely differently to my usual method for buying labels (which is to go the Seller Hub, click on Orders and then click "buy postage label" next to each item I need a label for.) I still don't really get why the front page eBay link takes me to a page for buying labels in bulk, while Seller Hub -> Orders takes me to one for buying labels individually (it's not really logical that they should give different results?) But now I know about it I'll just be extra careful to make sure I use the old method in future.