anyway im back to my unusual self. I think you should just house rule the hydra to *breaking out the MM's* have 2 more CR. but in actuality, i thinktheir basis on this decrease in CR (though a little too much IMO) was because if you look at each head, is onlly about a warrior in equality. But, if you took a 5-headed hydra, this would equate to a CR 5 creature becaus eeach head is about as tough as a 1rst level warrior, add fast healing maybe 6, i think that the head regrowing would probably just add to the EL. ( i would also rule that a downed head would count as a creature for cleave and allow them to attack only one or two other heads at most or if the head had attacked him last) it is basically to provide for the characters that have taken the improved sunder feat, though it does punish other for not. That is why God made house rules