Dizzee & Hurn-ia
This is an old chestnut that has been going on for years. The reason is quite simple.
NATS are used to taking people with no aviation knowledge, so they realise the information 'input' to the student is starting from scratch.
The other Colleges however, have found that most of their students already come from an aviation background, and so they already posess a lot of that knowledge. For that reason they've found that they can often compress the period - but you still do a '!!!!load' too (believe me - been there, done it).
Of course - if you don't have this knowledge then be prepared to work even harder on the non-NATS courses, because the standard that everyone has to reach - NATS or non NATS - is identical at the end of that first phase (and of course, all subsequent ones).