Irv - I would hazard a guess that it's probably because he/she can fly OK but doesn't have a JAR Class 2 medical? Whereas he/she will be able to hold a NPPL medical.
Incidentally, the reason why I personally won't be training pilots for the NPPL is that my RF is embedded in Class D airspace and we are required to train pilots to JAR/FCL PPL standard. However, if someone can't get a JAR Class 2 medical then we will train them to JAR/FCL standard, but they will only be able to hold a NPPL.