The Avatar is right.
But the way the code is implemented is even a little more complicated than that.
There is a PhysicalResistanceEffect that can be applied to both the player and enemies.
This effect stores a total amount of resistance, a max resistance cap, and
an effective amount resistance, which cannot go above the resistance cap, and is the amount displayed in the game.
The total amount of resistance which I first mentioned however may go above the cap.
When the player or an enemy gains resistances, both the amount of resistance gained and the new cap
must be given in the code.
The amount of resistance gained is then added to the total amount of resistance, the resistance cap is changed
if necessary, and the effective amount is then set to be the minimum between the total amount and the cap.
In the case of Stheno and most monsters, they gain some resistance and the cap is explicitly set at 90%.
In the case of the Paladin gold Challenge, the boss resistances are explicitly set to 95% and the cap to 100%.
I hope this is clearer like that. This is the way the game was coded, and it makes sense to me.
The philosophy is that they don't want the effective resistance going up to more than 90% because it would make them impossible to beat. But if you screw with Stheno however, that's your problem, and you will have to remove a lot of
total resistance from her before she starts losing some effective resistance...