As far as I know most r30s queue in the 50s bracket, where their gear has the advantage. Queueing in the 40s bracket won't net them any gear advantage whatsoever, however the advantage in terms of PvP abilities would still be there. To me, there could only be two discernable reasons for a r30 to queue in 30s/40s bracket.

1. They have one or more friends who want to play that bracket instead of 50s.

2. They've been getting their butt kicked in 50s bracket against other r30s and want a fight they feel they can win.