Of all the speculations and rumours about the 'real world' reasons for there being only 18 of the 20 legions recorded, dose anyone know the truth? And what theorys are there?

Were there 20 leed designers of rouge trader, but only 18 picked names for the legions/primarchs?

Is it a fluff reason? Or is the reason hidden in the fluff?

If anyone knows (and is allowed to tell ) please tell as I am sure I'm not the only one who would be interested.