It doesn't really mean anything of consequence. The important version numbers are those that read like 7.56 for example. The Detonator X number just sort of refers to the series that it is, you can use nearly any detonator version with the TNT2 series of cards.
The 'reference' driver is the one that nvidia release on their own web-site and will support any problems with it. Reference drivers are released as a sort of fall back if the manufacturers dont work. Manufacturers pretty much adapt the reference drivers in most cases. The un-official ones are not supported by nvidia.
WHQL certification means that it is basically MS approved, but dont let that sway your decision to use a driver.