[...] Personally I have yet to figure out how it does this though as pool doesn't report anything unusual...
The dev fee is generated by the miner actually mining to 2 different pools at the same time, 1 pool is the one(s) you setup and the other is the one the Dev hard-coded. Stratum has a function that allows this and the user never sees the hidden mining going on - no way to verify just what % is being diverted from their chosen pools.