Contribution ESA

Hi there I am looking for advice in relation to my current ESA claim, I was placed in the ESA support group after my transfer from the old incapacity benefit to the new ESA. after undergoing my work capability assessment I was awarded ESA and placed in the support group. I now receive contribution ESA payment. My partner is my official carer but also works part-time on a self employed basis. Every four months or so I am requested to send in details of the work she carries out, how much she gets paid, etc. The reason for this apparently is the amount she earns could affect my esa benefit, yet I have read that contribution ESA is not means tested so it should not matter what my partner earns, as it happens her part time work at best is only 4 hours a week and her earnings is £40.00 a week. If I am correct and contribution ESA is not means tested why do I have to keep sending in details of the work she does and how much she gets paid, when it should not matter...... any advice welcome. Thanks