So, one way to look at needing to be at 85% or less HP for it to be match/beat SR assault (and 70% being the threshold for SSR):

If you're spending the vast majority of a fight at full, or near full, health, life's pretty comfortable. You're presumably not at risk of losing, so you don't actually care about how much harder you're hitting, outside of time/vanity reasons. Anything can go in that slot, so it's not the most interesting case.

When you're spending more time in a fight further away from full health, that's when you start being under more pressure to be able to end the fight sooner. That's when you start caring about how much time you need to win versus the time you can hold out for. And the numbers just so happen to be such that this is when the SSR pride effect starts winning out.