Hi FedupofPW, and welcome to the Forum.
The major furore has been all about the boys until now, and that has been a huge shock to everyone - not least Admissions, but there is usually pressure on DCHS as well.
It is hard to say exactly why the situation can change so much between March and late August. If you look at this link, you will see the historic figures and, from last year's figures, how much change there can be: http://www.elevenplusexams.co.uk/11-plus-bucks-allocation-profile.php
Private school waiting lists, job moves, RAF Halton, indecisiveness - they all play a part in the shakedown between now and September, as do transfer appeals themselves.
Presumably the alternative school you have all been offered is Chesham High? It is a good school, and getting better all the time under the new Head.
One idea you might like to look in to if you feel strongly about a single gender school is a place on the waiting list for Beaconsfield High? For the first time in living memory (or thereabouts) they were "All Offered" on Allocations Day, which suggests that they may even have a few vacant places?
The success rate for Transfer Appeals for Grammar Schools is 15% - you will need very compelling evidence indeed to make your case.
Please read the Appeals Q&As first http://www.elevenplusexams.co.uk/11plus-appeals-questions-answers.php
, and post any questions about appeals on the "Appeals" section, not here.