I think the save that is granted by a fall, is more like a death saving throw (is there any rules on this, or is this just a hack?), and things that modify a death saving throw are different than those that modify a standard saving throw.
So, if a monster has +5 to saves, then it doesn't get that +5 when it comes to death saving throws.
That doesn't help us, but that would be my reasoning. But if you wanted to argue the other way, then I'd say that since your spell caused the shift which caused the need for a saving throw, then indirectly it is a spell induced saving throw.