I have always been led to believe AMH was an indication of reserve but not quality, although what Mel said is true too. Basically it's like bingo. Each month an egg is spat out (so to speak!) but as we get older the number in the barrel diminishes as the good eggs have been spat out earlier in our lives. So we are left with fewer of a lesser quality. Sorry for the bad analogy - my brain is diminishing too!!
MissB - egg quality is a current obsession of mine too!!! I am 40 in a month and I feel like my eggs are ancient!!!