I think there is a difference and that difference is accuracy vs distance.
An overhead cast directs the cast along a path that is in direct line from the angler to the target. That would be Joan Wulff's and Mel Krieger's method of teaching casting. Lefty Kreh casts a bit sidearm and so it is a cast in which the line approaches the target at an angle from the body of the caster. This sidearm motion along with body rotation allows Lefty to take his hand back further and this allows for a longer stroke length. It allows for a longer cast BUT because the tip of the fly rod is not directly overhead along the path from the body to the target, it is more difficult to be accurate.
If you angle the rod path at a slightly more vertical higher stop point or a slightly lower stop point with the overhead cast, gravity will still cause the fly to fall directly down toward the target. However, with a side arm motion, it the angle is off a bit, the fly will be off. Hence pinpoint accuracy vs distance is the tradeoff.
Regards,
Silver
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy