ESA and SDP

I hope this is ok to ask, since 2021 I have been receiving enhanced mobile and enhanced daily living pip. I contacted CAB to help with my transition to uc they asked why have I never claimed for SDP and I explained that I never knew about it. I have since contacted ESA who have stated that I should have been receiving it, they said I was entitled to backpay which I never expected, they put money into my bank on Friday but was a lot lower than I thought it would be, have I got the calculation wrong as I thought it would be from the date of my pip? Many thanks karen