My eldest DS is now in Y12 and the selection criteria have been exactly the same for all Bucks Grammars ever since he sat the 11+. There has certainly been no "dumbing down" of the selection process since 2005.
Once upon a time I believe that there was a "final tie-breaker" by score, but I believe it came after several other allocation criteria such as distance.
G55 is correct that the proportion selected via immediate qualification (i.e. scoring 121+) is consistent at around 30%, and successful appeals increase that to around 35%, but even the number of successful appeals is pretty consistent every year, so that can't account for it.
Urban myth, and possibly a degree of bragging: "It was so much more difficult to get in when my (incredibly bright) child qualified ..."