I finally get to study devastation for a bit. And I found what I think are reproduceable patterns in the "Devastation" phenomenon. At this moment, I noted 2 flavors: One is prevalent the other is rarer.

The Prevalent Type of devastation is that 10% of farm income (rounded down) is deducted. For example my farm income is 612 for Dimmidi, the devastation is 612x10%=61.2. Rounded down, that is 61.

The Rarer Type of devastation is present on two of my regions bordering enemy provinces: Lepcis Magna and Campus Scythii. The devastation minus here is higher by 10%, but here's the interesting part: they are higher by a factor of whole number (I've seen 4 and 6 so far) times the original devastation value. For example:

Lepcis Magna Farm Income: 408 denari
Devastation Value : 244 denari
408 denari x 10% = 40.8 denari
244 denari/40.8 denari = 5.98 = ~ 6

Campus Scythii Farm Income: 544 denari
Devastation Value: 326 denari
544 denari x 10% = 54.4 denari
326 denari/ 54.4 denari = 5.99 = ~ 6

Observations:
1) Prevalent type present in almost all regions with "devastation", except those with rarer type.
2) Rarer type is present on provinces bordering enemy regions ( Thrace (Tanais), Dacia (Vicus Venedae) and Numidia (Siwa).
3) Provinces with devastation always has a culture penalty.
4. Bribing rebels close by doesn't affect any variables in the rarer types.
5. No test yet done on prevalent type. But my guess is it won't affect it either.
6. I did a couple of tests where the Devastation value with factor 6 goes down by 4. There are also occasions where it stayed to 6.

Early Hypothesis:
There is an elusive, unknown variable there that factors in the devastation value. I'm still checking enemy borders, armies etc. but no definite answers yet. Though, I believe finding this factor will greatly increase the understanding this RTW feature.

In addition, there are other avenues that can still be checked and explored. For example, I won both provinces via prolonged siege. I eventually had to win by battle, but still, that could be a possible factor (since, the longer you stay in an area, the blacker the area becomes = farming penalty?). I also had alot of agents there, so it could also be a bug that reads agents as armies, thus destroy farming area if they hang around. There are many possibilities.

Anyway, I'll check some more into this later, but that's all the good news for now.