Its actually impossible to get resists to 100%.

Each additional resist piece takes that percentage from the the amount left.

Mathematically it looks like this: resists = 1 - ((1 - r1)(1 - r2)(1 - r3)...))

Lets say that you have a ship with 50% EM resists. You add a hardener that gives 50% EM resists.

So that would be 1 - ((1 - 0.50)(1 - 0.50)) = 1- ((0.50)(0.50)) = 1 - 0.25 = .75 or 75% resists.

While the hardener only increased your resists by 50%, it did double your effective EM HP. (Before the hardener it took 2 points of EM damage to do 1 point of damage to the shields, with the hardener it takes 4 points of EM damage to do 1 HP of damage.)

Various bits of your resist gear will also have stacking penalties (diminishing returns) too.