I read a post that
@Famine made in a thread a monthish ago that laid it all out; it was very easy to follow. I tried for a min but couldn't find it; if he's in a good mood maybe he will.
The gist of it, as I understand it, is it's 100% dependent on who you finish ahead of and who you finish below, as everyone in the lobby is stealing DR from everyone else. Let's just say there is a lobby with 10 people. Everyone except you is a C rated driver and you're an A+. If you finish 2nd, the C rated driver in first is going to take A LOT more DR from you, than you will take from the remaining C rated drivers in positions 3-10. Thus, you finished 2nd but lost DR.
That is another "risk" when lobbies are thin. When there is A+ thru B, should something happen to you (say there's a spin or a punt), you drop way back and finish back there, your DR is going to take a big hit; whereas if you finish with the other A+ guy, your DR won't gain much at all.
Someone can correct me if I am wrong here.
I did a Daily B on my 1 account today and ran in to this issue. Was starting P3 and there were A+ thru B rated drivers. I was thinking, if I bin this car it's going to hurt. Thankfully, I finished P2 (in the Mustang to boot)!