I'm going to guess that, after you enumerate all possible opposing hands along with all possible table cards, that the 7 gets paired up for a win more often tha

I'm going to guess that, after you enumerate all possible opposing hands along with all possible table cards, that the 7 gets paired up for a win more often than the straight lines up for a win. Also, due to their low card value, a 2-3 will pretty much automatically lose with no pairings, straights, or flushes. (And even then, it's a weak flush.) 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.)