[GAMEPLAY] Max magic pen cap at 40% ? or is it something else

So I see that having both a Void Staff (35% pen) and the mastery Piercing thoughts (7% pen) adds up to 40% (which should be 42% is what I'm thinking), so my question is, is there supposed to be a capped amount of magic pen or is it a bug or is there another way this is calculated cause getting Last Whisper (45% pen) and having the mastery Battering blows (7% pen) equals to 52% in-game which makes sense.