Base Critical Severity

I have noticed that when choosing the Critical Severity Modification on the off-hand weapon that gives you a 10% bonus to Critical Severity actually modifies the Base Critical Severity instead of adding the +10% I believe it should be. So at a 75% Base Critical Severity the off-hand actually calculates by taking and adding 10% of that value (75/10 = 7.5, 75+7.5 = 82.5) making it 82.5% instead of 85% (75+10 = 85). I am not entirely sure that this is a bug, or a developer choice. But, I found it strange that it was the only modification that affected the Base Critical Severity instead of adding to it as an additive modifier.