I am not flat Earth beliver, just to state.
So there are many models of Flat Earth, each with his own ideas and explenations.
So, one possible is that it is almost same as in globe case. Weaker magnetic field around poles mixed with whatewer sun spews in FE makes it. Another possible explenation is layers of aether. Poles are as far as you can get form sun, so if sun attracts/pushes aether away form itself, poles will be most dense/least dense. Then, you will have some sort of potential, and if you ad some particles into mix, you have sensiblish explenation
Hope it helps!