I have spellcraft and other spells perfected yet with one of the offensive ones my hits range from:

hits to DEMOs.

I know it is a spell learned very early but why such a broad range?

Just the way that spell was designed, probably.

If the spell already has a wide degree of variability, when you factor in people taking reduced damage via save vs spell, it could seem to have an unreasonably large range. Depends on the spell though, as the effect saves have on damage varies (I believe).