Because some of the skins at that cost don't seem to be any different from 750 tier skins, while some actually have some VFX changed which is usually 1350 tier.
Honestly I don't know what makes a skin qualify for the 975 tier, please help clarify.
This guide has all of "what makes" a skin in a certain tier https://support.riotgames.com/hc/en-us/articles/201752874-Guide-to-Skin-Price-Tiers
I believe current 975 skins all have some animations changed 750s are basically just a recolor 1350 has lots of animations changed
But they didn't really do this in the past, so only new skins that come out follow this rule.
Ignore older skins for the most part. The quality bar has gone up quite a bit over the years, and a fair few skins have aged poorly and no longer fit the standards of their price tier.
975 skins have a new model and texture, along with some new particles. They will NOT have voice processing, and the new particles they have may or may not have different sound effects.
1350 skins are typically set apart by more intensive particle effects and sound work. This frequently, but does not always, mean new voice processing. Often a 1350 skin means a new theme, rather than something that could be accomplished by the character simply getting a makeover but being basically the same character underneath.
Nowadays 975 skins have a new model,new/changed skill effects and nowadays tend to come with a new recall. The 1350 skins come with new voice processing or other special stuff.
