Normally when you apply a Penetrator mod to a gun, the stats display the Mod's stat + the gun's default stat.

However when you attach it to a Sniper Rifle, the Mod's stat completely replaces the Sniper Rifle's crit chance. Therefore a Sniper rifle with +40% critical chance before the mod, will only display anywhere from 2 to 4% with a Penetrator Mod.

This seems to happen to any tech sniper rifle, I have not tried it on other sniper rifles so far.