well, first of all your fSTR and pDif are wrong. WSC is most likely wrong as well, but to know the proper numbers I'd have to know what WS you're using.
First of all, fSTR formula changes depending on your STR - target VIT (that's dSTR). A rough estimate would be approx 4 STR = 1 fSTR.
dSTR range fSTR approximation
12 or greater (dSTR + 4) / 4
6 to 11 (dSTR + 6) / 4
1 to 5 (dSTR + 7) / 4
-2 to 0 (dSTR + 8) / 4
-7 to -3 (dSTR + 9) / 4
-15 to -8 (dSTR + 10) / 4
-21 to -16 (dSTR + 12) / 4
-22 or less (dSTR + 13) / 4
The cap for fSTR depends on your "weapon rank" which is floor(base damage/9) where floor means to round down. Given your example with damage of 40, that would put your weapon rank at floor(40/9) which is 4, which from this chart:
http://www.freewebs.com/vzx-01/fstr2.htm would mean your fSTR caps at 12 (the chart is of fSTR2 which is for ranged attack. Divide by 2 for regular attack) which happens when your STR is 44 more than the target's VIT.
Next pDif is is harder to estimate, because it's atk divide by def not minus, so you can't say much about it without knowing how much defense your target has (unless you're capped on pDif, then it's just the cap. Apparently according to wiki thee pDif formula was changed and there's still research going into what the new formula is, but this is the old one:
Ratio = your (r)atk/monster defense
Melee cRatio = Ratio - 0.050 x level difference
Max pDIF has the following formulas:Ratio Range Function Value
0 ≤ cRatio < 0.5 fMax(cRatio) = 0.4 + 1.2 x cRatio
0.5 ≤ cRatio ≤ 5/6 fMax(cRatio) = 1
5/6 < cRatio ≤ 2 fMax(cRatio) = 1.2 x cRatio
Min pDIF has the following formulas:Ratio Range Function Value
0 ≤ cRatio < 1.25 fMin(cRatio) = -0.5 + 1.2 x cRatio
if fMin < 0 → fMin = 0
1.25 ≤ cRatio ≤ 1.5 fMin(cRatio) = 1
1.5 < cRatio ≤ 2 fMin(cRatio) = -0.8 + 1.2 x cRatio
Again there's a number of different formulas depending on what value you get for atk/def - 5% of level difference. If you calculate using fMax as your pDif, that'll give you the highest amount that your swing would hit for (assuming no weakness or resistance towards your damage type like blunt vs bones etc). If you use fMin as your pDif that'll give you the lowest amount you'd hit for. pDif cap is 2.4 for non-crit and 3.0 for crit. A crit will increase your pDif by 1.0 so say if you're fighting something with high def and your pDif is 0.2 and you crit, you get pDif of 1.2 which is 6x the damage of a non-crit, as opposed to if your atk was capped for the mob's def, you'd go from 1.6(min)-2.4(max) non-crit to 2.6(min)-3.0(max) crit, which isn't as much. The amount you actually hit for is based on a random pDif somewhere between you fMin and fMax
As for WSC, let's take Raging Axe for example. It has a 30% STR modifier and fTP of 1.0 at 100%Tp.
WSC = floor (floor((A x A%) + (B x B%)) x α)
(btw floor means to round down)
Adding 11STR would give you
floor (floor((11 x 30%) + (B x 0%)) x 0.83) (0% because Raging Axe doesn't have a 2nd modifier)
= floor (floor (3.3) x 0.83)
= floor ( 3 x 0.83)
= floor ( 2.49 ) = 2
which at level 75 would be 0.83, so that would increase your WSC by 2 per hit
With multi-hit WS, the fTP is for the first hit, every hit after that has an fTP of 1. For dual-wield it adds one additional hit for multi-hit WS, so dual-wield Raging axe is actually 3 hits instead of 2.
In summary, your pDIF is Waaaaaaaaay off, because it's a number from 0 to 2.4 (or up to 3.0 if your crit) and 12 atk will not increase your pDIF by 12, it'll depend on how much def the target has compared to the atk you have already and would most likely increase your pDif by less than 0.05 unless you're fighting really low level stuff. fSTR and WSC are off pretty significantly as well, but there's no way to know your damage increase without knowing the mob's level, VIT and Def.
Edit: and yeah, what Den said as well, you get some atk with your STR as well, whereas atk only gives atk.