As I understand it, the amount of money you and your child are given per week to live on is paid out of the 'benefits pot'. The amount of Child Support your ex pays is calculated in a similar fashion by the CSA.
When you are in receipt of benefits your ex's payments top-up the benefits pot thus enabling them to pay you......it is an 'instead of...' payment rather than an 'as well as...'.
They are now clawing back, from him, all the money that they paid to you. That way, you have received your entitlement and he has fulfilled his financial obligations.