Not entirely sure about why JHPs would cycle better in curved vs. straight mags, but I do have an idea about why some mags are curved -
If you look at the brass bullet casings, they are not perfectly cylindrical. They taper a little - smaller at the front than the rear. This helps with extraction. If they were completely cylindrical, they would be pressed against the chamber the full length as they were pulled out. With the taper, after they are pulled rearwards just a little, they get significant clearance from the chamber walls. A pistol round will need less taper than a rifle round, simply because of the shorter length. The shorter the case is, the less of it is contacting the chamber wall during extration.
So... if you lay a bunch of ammo side by side, they won't line up in a straight line. They start to form an arc. The mags are shaped to allow for this. There are a lot of factors that determine how much, if any, curve is needed - a greater taper will require more curve. A low-capacity magazine might not need to be curved as the stack of ammo may not be tall enough for the curvature to really become apparent. A single stack magaine might need more taper than a double stack of similar capacity.