I think, but have not verified, that TTN might use RX2 when the uplink uses SF12. For that, TTN’s SF9 in RX2 needs less airtime than RX1, as RX1 uses the same SF as the uplink hence would need SF12. (And SF12 need twice the airtime of SF11, which needs twice the time of SF10, etc.) If true, then it might also prefer RX2 for SF11 and maybe SF10. See also:
(Full discussion in the outdated Why is RX_SLOT 1 not used?)