I didn't apply specifically for ESA I was on income support and was told that I need to change over to ESA and I would need to go for a assessment, I will give them a ring tonight and ask them why they have said this

Hello I went for my assessment in January 2012 for ESA (I was previously on income support) I was told that I didn't score enough points to claim ESA so I put in an appeal, while I was waiting for my appeal date I was claiming £71.70 per week which I believe is assessment rate. I won my appeal on 15/05/2013 I got ESA with the support component and enhanced disability premium. It has been sorted and I have received my back pay of £3503, I had a letter on Friday saying that I had been claiming contribution based ESA for the year 2012/2013 and I am to be taxed on £3950 and if I disagree I have to write to them within 60 days, I have no idea what this is how could I have been claiming contribution based ESA I was on assessment rate and I came from income support I am going to phone them but I just wanted to understand what was going first Many Thanks