Just quoting some random dataminer from battle net forums, who may know the real details or may have source = made it the fuck up
The claim is that the HP cap is different for different classes of demon, as the patch notes indicated, with Lister & Hephasto both having a capped base HP of 20000 + 1000 per addition player up to 27000 in P8
whereas spearwomen have cap of 13200 + 660 up to 17820 in P8
When do these caps apply? Does any of that actually track?

Spearwomen have an HP range of 80-115% of monlvl, with 7447 monlvl in TZ96 (used for level 96 trash and 99 uniques alike, base HP is before the +3 levels)
that should be 5957-8564 for trash mobs in P1, 11914-17128 for uniques.
If the numbers from datamining are accurate, and if they indeed apply before P8 scaling, then getting a unique would fully double the HP
If the cap is applying after P8 scaling, then both trash mobs and uniques would always have 17820 hp since they all have way higher HP than that (26.8-38.5k for trash mobs)
This is something we should be able to test for: Simply binding a trash mob vs unique in a P8 game and see whether its actually ~2x the tankiness. Wouldn't be the easiest thing to find replicable DPS sources without much variance, at least in PvM, and the HP ranges still make it tougher to lock down, but eh