Tornado Shot Tooltip is confusing

When leveling Tornado Shot, the base damage is supposed to increase from 90% at level one to 109% at level twenty.

I'm currently playing a (Elemental) Tornado Shot Witch and when I just leveled Tornado Shot from lvl 14 to 15 the DPS didn't increase at all (4055.2 dps before and after).

Now, is it possible that this base damage increase only affects physical damage or is it not displayed on the tooltip no matter what the damage source?

I encountered the same thing with lvling the gem from 18 to 19.
Since I also had another lvl 18 one I tried to figure out the reason.

It seems that the GMP support gem causes this (haven't tested with other less modifiers like LMP or chain). Without GMP the lvl 18 and lvl 19 ones had a top end phys damage difference of ~30 (per hit, at ~3k total top end phys dmg). With GMP in the dmg got reduced as it is supposed to be, but both lvls had exactly the same top end phys dmg (~1.8k) and also the same tooltip dps.

My guess is that this is because of rounding and occurs every second gem lvl (as in your case at 14/15 it is the same).
Maybe the base damage gets multiplied with the GMP less modifier and then rounded:
- lvl 13: 102% * .5 = 51%
- lvl 14: 103% * .5 = 51.5% rounded to 52%
- lvl 15: 104% * .5 = 52% rounded to 52%

Seems like a solid explanation. It's just very counter-intuitive to see your damage not increase at all even if it supposedly should.