meant to reply sooner paul, with regard to multijet's. This is what I think but I could be totally wrong since I am too lazy to look it up and check.
I am guessing that the multijet uses some special injector that can spray in a little or a lot of fuel. Since there is only a short time window in which to get fuel in, most injectors just squirt fuel in for slightly longer, but then some comes before/after the critical window and you get smoke. I would guess from the name that multijet injectors are able to put in more fuel when required in the shorter space of time. Instead of putting the same amount in for longer they would be able to just get more in in the same time frame.
this is all a very haphazard guess.
