Oh, I don't know that the person I talk to has a phd, but she is a recent graduate and is a veterinary canine nutrition consultant (I think that's her title).
The way ash content was explained to me was: ash content is the minerals that are left over after the kibble has been incinerated. The reason that it is bad is because it is unbroken-down minerals left for the kidneys to process. I was told that water dilutes these excess minerals, though, and makes it easier on the kidneys to filter it.