18% below 50%

I just found a falchion with 18% damage while health is below 50%. I thought the max was 15?

Max for above 50% health is 15.

15 above 50 is max.
20 below 50 is max.

The two Damage + mods that have a max of 20% are (while health is under 50%) and (while hexed). All other forms have a max of 15%.

Didnt they upped the above health to 20% aswell in the pvp weapons?

Nope, max for above 50% mod is still 15% more damage.

positive mods that benifit you (or that you have control over) are 15%
negative mods that you either have no control over or are bad things happening to you are 20%

If they add new mods such as "While suffering from a condition" they will be 20%
If they add new mods such as "Vs a bleeding foe" it will be 15%