Hi, David,
Thanks!! :-)
After I wrote that, I reminded myself (for much different reasons) about a movie I saw in the early `80's, "Hardbodies" and found it on YT, so I watched most of it. (Sadly, whoever posted it (in 9 parts) didn't post parts 5 & 6.) In the very first scene, Scotty is just finishing up having sex with his GF, Kristi and she's lying flat on his bed and I noticed that hers didn't quite do that! I'd say she was a large A/small B. While on her back, they just kinda got flat.
I think if you look at it this way, you'll see why they're going towards your arms. Your nipples (for lack of a better way to put it) are kinda like a string attached to the opening of a balloon and to the "nipple" of the balloon. If you could set up a water balloon to hold a string like that, you'd see what I mean. And, although it always wants to point straight ahead, the rest of the breast tissue (the water in the balloon) wants to follow gravity. So, as your chest is kind of rounded, when you're on your back, the inner tissue wants to roll off your chest, but your skin won't let it. Although most of the breast goes in that direction, the nipples are still pointing straight ahead.
I'm sure someone can put it in a more scientific way.
Take care!!