If you believe some folks who are well connected in the industry some of these guys just buy whatever the best weed they can find is at the time in Amsterdam and then enter it calling it their own.
Yes, that's been alleged too.
In fact, one of the High Times senior staff actually bought a bunch of top level Super Silver Haze buds, then hired actors to play breeders, enter this weed into the contest, make appearances, talk smack about their strain, etc. In a bit of deliberate irony, the name of the fake strain was "Ooki Kabuki", a reference to the bizarre Japanese theater of the same name.
The idea was basically just to tweak the egos of the real growers there, but also to sort of point out weaknesses in the contest to make it better.
You can see it documented right here, at around minute 37 in the High Times cannabis cup video. . .first the actors talking smack, then the editor in question explaining what happened and why. You'll probably have to watch the whole video to get the full context:
http://www.youtube.com/watch?v=xCpuJfSEF6o
If you do watch the whole movie, in a different segment, you'll also see exactly how the big breeders "buy" their cup wins with direct to-the-public marketing, with the High Times staff not only admitting that this happens, but kind of shrugging it off, saying eh. . .if that's what the public wants, that's what they get.
Bottom line for me is. just that:
a. In any given contest, the winning strains simply may not be the "best" ones entered, even in the contests where the judging is closed to the public.
b. Even if the buds that win are legitimately excellent in their own right, the seeds in the pack carrying that label you buy from the award-winning breeder simply may not be the same genetics as the stuff that won the contest, and
c. Even if the genetics in your pack are the same ones that won the contest, there is no guarantee that you'll be able to use them to grow the same quality weed in your setup. There can be phenotype issues, hermies, and simply a poor match between your grow setup and the strain in question