I can only explain this, my pensions in UK include 2 private pensions and the Uk gov pension so I pay a bit of income tax because it's only just over the threshold.
If Thailand baht goes below 30 to £ I would not have to pay.
My extension visa money left in the Thai bank gets interest which is taxed by Thailand I don't bother to get it back which I could if I wanted that's how I understand it, if this thread was in the correct forum you would get a better answer.