I was doing some solver work and noticed this counter-intuitive result that I could use help understanding. The hand is a SRP BTN vs. BB. Board Q942 rainbow. Here's the action:

Flop Qc 9d 4s (5.5 BB)
BTN cbet 1.8 BB
BB x/raise to 7.3 BB
BTN calls

Turn Qc 9d 4s [2h]
BB bet 15 BB
BTN?

BB turn barrel range
BTN turn range vs. BB bet

I was really surprised to see the solver preferring to call 4x over 9x as the BTN player. At first I thought the 4x hands were preferred because they had an overcard like A4/K4 vs. the 9x hands like 97s, 96s. However the solver chooses to call and raise even the 4x hands like 64/54/43 over 97/96. Next I looked at the BB turn barrel range to look for blocking effects that would make BTN prefer calling with but nothing jumped out at me. Finally I looked through many other turn cards to see how BTN strategies shift, but consistently across almost all turn cards (aside from a 9 turn) the BTN was calling and raising much more with 4x and 9x.

Could the issue be that the 9x hands like J9 have kickers where if they make 2pair, villain will make some straights with their range so BTN is more likely to get coolered and lose a big pot? When the 6x hands like 64 make 2-pair, villain won't make any straights so BTN's EV is higher. This is the best explanation I can come up with.