I believe it is a factor of how likely they think your ride will get matched.

I have been riding uber every day for the last few weeks after a skateboard crash. Pools from home->work are almost always ~$7.50 and pools from work->home are almost always ~$2.30. I believe it is because i work at an uber hot-spot and live in an uber dead-end.

I use it every day. I've used it every day for nearly a year. I almost always get the promo discussed.