Ok so the reason you still went to -75 even with 150 and 85% res, is because your actual resistance from gear/charms didnt change.
In order to maintain the 85% you would need 235% in order to keep the 85% at -100 resist from Lower resist (or other sources like conviction.)
If you do not have conviciton or lower resist affecting you (in pvm) you will maintain the 85% resist.
In PVP there are also items, and jewels that can lower resistance in addition to conviction and the lower resist curse.
Basically the stat (increase resistance by X%) is simply that. It raises the cap from 75, to whatever the cap is raised to.
Things like Water Walks boots, Thundergods Belt, Guardian Angel Templar armor, Hot Spurs boots, Nokozan Amulet, Snowclash, are among a few that can raise the cap.
Even a paladin with points into a resist defensive aura, like Resist lightning/Fire/Cold, can increase the resistance without these items with enough points invested in them.
Facets raise your %skill damage, as well as lower enemy resist %
Things like Infinity, Crescent moon, Flickering Flame, Doom, Plague, can lower enemy resistance.
This is where characters might use skills like salvation (from paladin), natural resist (barb), Fade (assassin).
Or they may seek out extra resist in charms or jewels, or runes and gems.
Even things like 4 perfect topaz, in a paladin shield, can net tons of extra lightning resist, to deal with conviction, from things like uber mephisto.
To get extra saucy for a paladin, with a pretty rare/expensive item, one could use a jewelers sacred targe of deflecting that has the 45 resist auto mod, with 4 perfect topaz in it. You could get 205% resist to lightning.
It’s not really needed, but can be done.
Edit: The math is probably wrong, but I’m not checking it. You get the idea.