It seems as if the points are purely additive and then the resistance is computed from the point total according to some formula.

Based on the values you published (20 pt = 20%, 50 pt = 40%, 70 pt = 50%) a piece of gear granting 33% resistance should have around 35 points.

If that's the case, then two such pieces of gear would be worth 70 points, which is 50% resistance.

Perhaps not coincidentally that seems pretty close to what a human would have experienced pre-change if he could somehow make himself "double resistant" (e.g. a character with resist_positive and resist_metals being struck by a metal weapon with a positive attack).

33% resistance meant you took 2/3 damage. 2/3 * 2/3 = 4/9, which meant you'd be taking around 44% of the damage you normally would have. Post-changes you're looking at taking ~50% damage instead of 44%.