Shadow Partner Confusion... Fourth job buff?

Guess so. In that movie of ShiKagem, at 0:07 he hits 12101, 10894, 10203, 6473, 5828, 5458. Meaning SP = 0.53491, 0.53497, 0.53494. Times 2* this is 1.0698, 1.0699, 1.06988. Always very close to 1.07 (though less since it rounds down 1 dmg), which is the passive damage increase of boomerang step at lvl 3 which he very likely has (since he's lvl 121).

*proof:

 Spoiler

Say x is the normal dmg, and y the dmg increase. Then x*y is your damage. Hypothesis is that SP's dmg is 50% of the normal dmg, but again *y, so 0.5*x*y*y. To have y, dividing SP dmg to normal dmg you get 0.5*x*y*y / x*y = 0.5*y. So to get y, just do *2.

Hmm to go even further, this means the 25% passive increase is not 25%, but 0.66*125%+0.33*125%^2 = 35.4% increase xD

This means:
Steal = 3*1.25 + 1.5*1.25^2 = 609.3% (instead of 562.5%)
SB = 6*0.9*1.25 + 6*0.45*1.25^2 = 1096.9% (instead of 1012.5%)
BoT = 3.95*1.25 + 1.975*1.25^2 = 802.3% (instead of 740.6%)
Assaulter = 3*2.9*1.25 + 3*1.45*1.25^2 = 1767.2% (instead of 1631.3%)