Why are they allowed to take anything I earn over £170?

I have been paying £80 p/w CSA made up of £35 CSA and £45 arrears. (on a deductions of earnings order)My son reached 18ys in Aug 2013.

The child maintenance stopped but the arrears continued. It was decided that the arrears should be paid at the same amount as previously. Whatever I earn, I only ever get to take home £170 p/w. Any overime etc goes to CSA. I have been paying all different amounts from my wages, ie £95 p/w to £130 p/w.

Surely the amount I agreed to pay should be the amount. Why are they allowed to take anything I earn over £170? I am really sruggling to pay my mortgageand my bills, but there is no discussion on this.