[brucepick]

I think there's something else going on here:
Car is currently running on 89 octane due to good wife believing she heard knocking. "OK Honey, I will put in only 89 octane till we figure out what the noise is".

Edit:
I think the higher octane is enabling greater ign. advance - and the fuel management map limits lean burn when there's more advance.

Higher octane will allow more advance, as the knock sensor dynamically contributes to advance settings. I suspect the ECU is programmed to disallow lean burn when advance exceeds some predetermined amount. Of course ECU is designed for 87 octane but with 89 the advance will be more than generally expected so lean burn is disallowed more often.

This would fit with less lean burn occurring in wet conditions - the water 'mist' in the intake air also reduces knock, allows more advance, and so probably places more restrictions on lean burn.

I'll be testing for this by using different octanes, but at ~475 miles/tank that testing will not be rapid.