pDIF fluctuation formula is more complicated than that, and works at different tiers. That's why pDIF min and max don't scale at the same rate.
I gathered some pDIF fluctuation data and it looks like this:
cRatio = 1.130
pDIF(min) = 0,83 ~ pDIF(max) = 1,48
cRatio = 1.175
pDIF(min) = 0,86 ~ pDIF(max) = 1,52
cRatio = 1.466
pDIF(min) = 1,00 ~ pDIF(max) = 1,90
cRatio = 1.787
pDIF(min) = 1,34 ~ pDIF(max) = 2,23
cRatio = 2.145
pDIF(min) = 1,75 ~ pDIF(max) = 2,63
cRatio = capped
pDIF(min) = 1,85-1,87 ~ pDIF(max) = 2,700-2,750
It's hard to draw an exact formula for pDIF fluctuation with this low data, but it seems to be something like
if 1 < cRatio < ~1.75
pDIF(max) = 1.3 * cRatio
pDIF(min) = 0.7 * cRatio
if ~1.75 < cRatio < cap (2.2~2.3, haven't tested yet)
pDIF(max) = 1.23~1.25 * cRatio
pDIF(min) = pDIF(max) - 0,9
This would mean that when cRatio is between 1 and 2, pDIF fluctuation average value is exactly cRatio, but when cRatio exceeds 2, pDIF fluctuation average value is a bit higher than cRatio.
Considering your data it seems that Yukikaze has a 1.5 Ratio multiplier and Kasha 1.65 ish, which agrees with my new and more accurate yuki/kasha tests. And Gekko is definitely 2.
I have never fought JoL as DD (always been tank), but the only reason you get that kind of dmg from Kaiten on him is an extremely low pDIF. According to Wiki JoL is only lv83, so since the correction to ATK/DEF Ratio isn't huge, JoL must have an insane defense. Do your melee hits do in the order of 0-30 damage too?
As for Fafnir, his level is rather high (88-90), thus you get a big penalization on your ATK/DEF Ratio, even though his defense is probably not that high.
Take the "ATK > (1,5 + Correction) * Mob's Def" formula I posted above, which isn't completely accurate till I find out/someone tests ATK/DEF Ratio cap and that I have estimated at 2.3;
on Fafnir, assuming he has about 370 defense, Kaiten will do more than Hagun Gekko only if your atk is higher than (1,5 + 0,7) * 370 = 814. And this is due to his high level, rather than his high defense.
On JoL, assuming he has about 500 defense and his level is really only 83, atk must be higher than (1,5 + 0,4) * 500 = 950 for Kaiten to outperform Hagun Gekko.
On something like Greater Colibris, which have 320 defense not counting Dia2 and are LV82, you need more than (1,5 + 0,35) * 320 = 592 attack
Rather than a math error, Kaiten most likely doesn't have the "attack boost" or multiplier that many other single hit WSs have (tachi YGK, spinning slash, ground strike, etc) which is rather dumb on SE part imo..
if you want to test it, parse 50ish or so Kaitens on DC-EM steelshells in Boyahda keeping your STR and attack constant (and in the 400-500 range), and we can see whether Kaiten has a low attack bonus or none at all.