Yes. As you know, it isn't illegal to enter the country on a dinghy (say) provided you are here to apply for asylum and provided you do so at the first reasonable opportunity. Once your application (and any appeal) has been dismissed you no longer have a valid reason for being here.
The reason, I am told, for this policy of simply letting them loose is that the Home Office simply do not have the resources to hold the failed asylum applicants pending deportation. So they basically do what they need to do to make it someone else's problem.
So I suppose another way of explaining your 75% figure is that it is Home Office policy for the other 25% to become part of the black economy.