I thougt that a 2 and a 7 were the worst, but here a 2 and 3 has -35.4% win%-lose% while 2 and 7 has a slightly better -30.8% win%-lose% (second diagram). Any idea what the difference is?
23 offsuit is the worst hand against a single opponent because the likelihood of needing a straight or flush to win the hand decreases and the penalty for having a lower highcard is higher.
If you look at the final diagram, they are almost equal in average expected payout, with the 2-7 dropping value much more than the 2-3. I would expect this is due to the paired 7 not being very strong against more commonly played hands with higher average card value. (In other words, 7 doesn't mean as much when the average opponent has cards higher than a 7.)
The reason is entirely high-card strength vs all the hands including cards less than 7, like 65, 64, etc.
EDIT: And the value of pairing the 7 vs pairing the 3, again where high card strength matters--6X pairing the 6 won't matter as often.
Given there's only two hands, they are somewhat likely to showdown unimproved, not making a pair or better. If you analyzed ten handed tables, 2-7 fares the worst.