It may be because for people on the OAT/NJE scheme, 20k euros of salary for the first 5 years is tax free because it is classed by the HMRC as "repayment of training expenses". The fact that you may not need to have borrowed anything to do the course is irrelevant - it is a tax break that the UK Revenue allow if you have a job signed up prior to forking out for the training necessary for it (hence 5yrs x 20keuros = c.66k sterling, ie cost of OAT course). One little bonus that the self-sponsored guys & girls will always miss out on sadly.
I think you may be right RedSnail regarding the per-diems; OAT included them in the presentation I saw on it all, though I haven't seen the exact numbers they're talking about here.