Couple of points about waiting lists...
If it's a council list, I suspect you'll find that queue-jumping isn't a frequent occurrence. As it's a public service, they have to be seen to be fair to everyone, irrespective of keenness.
Even if there are a lot of people on the list, it doesn't always mean that you will have to wait for that number of plots to come vacant before you get one.
I manage a list which has had as many as 60 on it; I was estimating a 3-5 year wait (worst case scenario). It's now down to 43, and people are waiting no more than about 18 months. Still a long time, but not as bad as it was. In 2007, I added 36 to the list, allocated 24 first halves (and 1 second), and 22 people withdrew from the list without being allocated a plot.
One reason is that, as Trillium wrote, people move / change their minds / get a plot elsewhere etc without bothering to let the person in charge of the list know.
However, I think the main reason the list is dropping in length here, is because for the last 18 months the council has had a policy for all its sites with lists, that it will only let out half plots to new tenants (approx 150 sq yards on average). This gives as many people as possible the chance to start gardening as soon as possible.
People often think they want a full plot, but then discover that it's more hard work and time than they bargained for (especially families with toddlers, who overestimate their kids' boredom threshold...), and are grateful that they only have a half plot to tackle.
If they garden successfully for a season, we can then look at increasing the space they can garden.
Also, existing tenants of full plots can't take on further plots now unless and until there's no one waiting for a first or second half plot. (Existing tenants with more than one plot can keep all their plots, we're not chucking them off!).
If your area has long waiting lists, perhaps this could be discussed with the council allotments officer - it doesn't take a change in tenancy agreements, simply a change in policy. And the council actually gets more in rent here - because a half plot costs more to rent than half a full plot (if that makes sense!).