I held off on this for a while and I will accept perhaps I am just getting the raw end of RNG. But after multiple save failures I figured I would ask the question, particularly given I have felt this in the past as well.

Since getting some -117 save spell it seems an awful lot of spells are working first time:
- Curse
- Blindness
- Forget
- Sleep

I seemed to save far more when around the -70 mark. (Only recently having the extra -50 odd saves jump). There isnt some wierd thing where saves does a lap of victory over -100 and that extra 100 isn't considered.

Obviously this is a bit of a "gut feel" and such things are hard to quantify when edges are other things come into play.

Took a brief look, everything seems to be mathing right.

It's more likely you're being cursed by the RNG gods.
Did you forget to tip them recently?