I know of one reason why this could be happening. All the grammar schools have provision in their admission policy for dealing with children who move into the area after the normal Year 7 admissions process has started, including those who arrive intending to enter a later year group. I'm quoting below from the Pate's admissions policy http://www.pates.gloucs.sch.uk/images/admissions/2014_Admissions_Policy.pdf
, but I believe that all the other Glos grammars do something similar.
The first bullet of section 3.2 on the second page explains that they try, where possible, to sweep up genuine cases of DCs who could not make the normal test date into a second round of testing before the end of November. These DCs get put into the normal March allocation and should not be part of your problem.
However, the second bullet point covers DCs who unexpectedly move to the area later than this. Such late arrivals can take some form of test and get a score, but they will not automatically get a place at the school, no matter how brilliantly they do because the school is bound to be full. If the late arrival scores higher than anyone on the waiting list they will be put at the top of the waiting list. Some late arrivals will not qualify and others will score similarly to DCs on the waiting list. In these cases, I understand that the late arrival is slotted into the waiting list at the position determined by their score and other relevant admissions criteria, i.e. distance from the school or whatever the admissions policy says.
I do sympathize because it is frustrating for those of us with DCs on the waiting list, but I for one would not want the schools to batten down the hatches and deny the opportunity to someone else's brighter DC just because their parent got relocated. In a year or two, I could be looking at this problem from the other side.