Reports are only as good as the data used in them.
In this case, it's compromised by the greater ability for wealthier abusers to go undetected -
Fewer neighbours close by to report,
The ability to come across as absolutely plausible and a veritable Pillar of the Community when saying 'Of course, she has mental problems, you know, it's all so very difficult',
The greater number of options for escape when a victim has access to money to be able to leave, family or friends with room for them to stay,
No problems buying another property or taking out a rental contract,
No difficulty affording short term childcare options rather than relying upon the partner or a member of their family,
Being able to buy food, clothing and pay for travel expenses instead of having to wait for a UC application to go through after leaving,
Less imperative to report the abuse so that the local authority housing department will accept an application,
A greater reluctance to report because 'Nobody will believe me, we have such a lovely home/he would be suspended from work and everybody would know'
'We're not that sort of family'
'He's very stressed in his high power job, it's not really abuse'
'It'll be seen as a way to get full custody/house/punish him because he has friends who are lawyers/can afford the best lawyers/will come across really well in court'
and a bit of 'But then I'd have to go on benefits, I can't do that to the children and he'll take them away if I have to live in a tiny flat on an estate miles away from school and can't pay school fees because I've had to leave my job'
I think those factors significantly impact the data relating to wealthier households.
And there's also the consideration that the people doing the research are inherently more likely to be of the higher income group themselves - so they wouldn't necessarily think of other metrics beyond tracking income level and number of police reports to measure the factors around rates of abuse.
You can also add in potential biases introduced by the purpose of the research - is it to justify reductions in support in wealthier areas, to increase support in poorer ones, to draw in sympathetic higher income people to make donations because they see 'children in poverty due to abuse', rather than just terrified women and children who they might see as perfectly capable of leaving due to income? Is it in the interests of who is funding the study for violence to be seen as something that happens to poor women, not theoretically (ie, taking into account financial abuse) or actually wealthier ones?
I don't think the rate is the same, as the lower income group will include a lot of other contributory factors, such as mental health and learning disabilities - but I think they could be closer were other methodologies also utilised.