Considering ice storm is Evocation and sleet storm is Conjuration, I would chalk it up to ¯\_(ツ)_/¯
I think a lot of the "confusion" is the spell names themselves. There is no "
IUPAC" nomenclature for spells.
Ice Storm might be better as "Hailstone Swarm" (like metor swarm) since it is an evocation - it brings forth hailstones from 'nothing' (well, from magical energy). If the spell changed the weather to a literal ice storm, it would be transmutation. If it brought forth (called) an ice storm into being from "elsewhere" (the para-elemental plane of ice, Stygia, the north pole, etc) it would be conjuration.
Sleet Storm might be better named as "Create Sleet" since it is a conjuration.
Whats difference between conjuring ice and creating it from nothing? In these cases, a conjured Ice Storm (hail storm really) has a kinetic component that can't be contained in conjuration. Sleet storm brings forth a literal sleet storm, albeit very intense, but sleet isn't damaging in and of itself.
A conjured effect like sleet storm could be used as a basis for a real weather sleet storm mechanical (game) effects.
An invoked effect like Ice Storm is not appropriate to model game effects. It is nothing like a literal ice storm.