...Another possibility - wrist angle? Based on the angle of the brifters on the bars...

+1

I had a similar issue with my bike when it was new. When on the hoods my wrists were bent upwards. LBS simply rotated the bars down a little, and dropped the brifters a bit lower so that when on the hoods my wrists were in a much more natural angle.

Numbness went completely away and never came back.