Some of the oil workers (on equal time that is) have their tax paid or are paid an additional sum equal to their tax burden to equate it to tax free. Others are not trying to be tax free; they just get paid a lot more than elsewhere which makes it worth it.
The ACN Brits are probably mostly illegal but taking the chance they won't be caught by the system (as indeed are many of the BHNL Brits!)
Can't argue with your sums though!