Corrupting skill gem does not apply +1 level

Hi.

To make it short, I'm using Shockwave Totem build. It is scaled with gem level so I tried to max it. In theory max level should be:
-gem lvl 20
-gem corruption +1
-1st weapon +7
-2nd weapon +7
-amulet +3
-gloves +2
-Physical Mastery support +1

That's 41 total.

BUT Physical Mastery does not add another +1 to lvl 40 skill. It stays at 40.
It adds another +1 to lvl 39 skill, making it 40.
Seems like 40 is cap (?)

According to skill description it adds to 41 tho...but shows 40...
https://drive.google.com/file/d/1JBAC0PGDsY002RO9G8bRFple1FQzZrzG/view?usp=sharing

To show it better:
https://drive.google.com/file/d/16DdSmb5nnoEMIbcYoax4Ps_xahxgHe1t/view?usp=sharing

Above is corrupted lvl 21 skill gem.
Below is regular lvl 20.
Without +1 support.
Same damage :/