Domestic violence situations are nasty. And as someone posted above, it's hard to compare one to another - each one has their own inner dynamics. I will absolutely hold my judgement until the facts of the case come forward. There are three sides to a story.
There are two sections that can be used in domestic violence crimes: The felony section - 273.5(a) or the misdemeanor section - 243(e)(1). If a injury is visible, no matter how slight (from slight redness to full-on lacerations/bruises/etc), the felony section is applied. If an accusation is made and there are no visible injuries, the misdemeanor section is used. One thing to note, is that in domestic violence accusations, the 836PC section states the officer SHALL make an arrest in these circumstances. It's one of the only sections that mandates an officer make an arrest when it is warranted.
I know I read earlier in this thread about the Bruce Miller case. If I remember correctly, he was charged with the misdemeanor section for domestic violence.
EDIT:
I thought I would add this last piece, as I have investigated several domestic violence cases. There have been times that it was a he-said, she-said type of case, with little to no unbiased, supporting facts -- and I did my best to find the aggressor in each case and make an arrest. This was done knowing full-well that the case was very weak and would probably be dropped by the DA's office.