Based on the mortgage amount in your first post - €545.69 per month, this works out at €125.92 per week, not €136.42. Weekly payment is calculated as follows - 545.69 x 12 / 52.
Your total income (434.60) is 68.70 above the SWA rate for your family, which is 365.90.
Your entitlement appears to be 125.92 - 68.70 - 24 = 33.22 pw.
I suggest that the 39.20 you're actually getting means that the CWO hasn't adjusted the mortgage to take account of the increase in minimum contribution which changed from 18 euro to 24 euro from June 2009.
If you've any queries about the amount, it's best to contact the CWO themselves, just to make sure that they've got all the up to date documentation for you.