People flying nonrev do not have to list for the front cabin. There are reported to be extra fees for clearing into F/J, although I don't know what magnitude.
So here are a couple possible explanations (with arbitrarily chosen numbers for the list position):
--
SBY
8. XXX, A (1) [rev, standby for earlier flight]
9. XXX, A (2) [nonrev]
UPG
16. XXX, A (2)
--
SBY
8. XXX, A (1) [nonrev, likes chances for UPG, willing to pay extra fees]
9. XXX, A (2) [nonrev, doesn't want to upgrade]
UPG
16. XXX, A (1)
--