Thanks for the replies, guys. It's pretty unlikely that I mis-measured the ingredients, just because of the way they were packaged -- even if I had screwed up the tare function, I couldn't have been off by much, because I know how many bags I used.
So the unmixed top-off water theory seems more likely, though that also surprises me. I would never have guessed it would be so hard to mix! This batch I happened to make a real point of aerating vigorously. I shook the carboy around until my arms couldn't take it anymore, and then my buddy shook it until his arms couldn't take it (kind of a contest
). Would you guys recommend actually stirring the post-top-off wort with a stick of some sort?
I've always just added my top-off water by pouring it through a funnel on top of the strong wort. Maybe I should try racking the water in through a tube that runs to the bottom of the wort, to ensure better mixing?
Edit: BTW, the airlock is bubbling quite furiously, and there's a nice krausen, so whatever happened the yeast (Safale S-04) don't seem to mind