Thanks, Hubert, for replying so quickly. Re-checking the indicated sections of the manual, what I'm seeing seems consistent with that.
So, yes, SeaMonkey/James Taylor. I buy that it's not a bug. Or if there is a bug, it's in how the S-hotkey displays supply for hexes actually occupied by HQs. It does seem a little odd that HQs are selflessly distributing more food, fuel, and ammo to nearby units than they are keeping for themselves. Doesn't sound like almost any military brass that I've read about.
My underlying concern is game balance when we get multi-player. Given the terrain and the fact that Axis-occupied Towns in the USSR never seem to improve beyond supply level 5, this feature slows Axis HQ movement significantly throughout much of Russia. So Barbarossa and subsequent German offensives very quickly run beyond their supplies. I know they did IRL, and they would in the game would even if HQs benefited from their Supply Distribution value instead of the intrinsic supply of the hex, but I think this feature of supply mechanics really penalizes offensives in Russia.
This hasn't been much of a problem against the AI, which is good, but not that smart about using lots of "speedbump" units to delay the Wehrmacht. Slow HQs may prove more of a problem when facing a human playing the USSR. I fully expect to fail to get even to Smolensk by winter of '41 when facing any reasonably competent human, and slow HQs will play a big role in that outcome.
But I'd be happy to be proved wrong!