Reduction in esa payment

I moved over from esa to universal credit last year and I kept my contributions based esa payment of £600 per month.
However I've just received a notification from uc that my uc payment has increased by £120 a month, however my esa contributions based has gone down by the same amount.

Can anybody explain why this has happened, I am scared that they are going to stoo my esa payment

Thanks

Thanks for the reply.
Yes the reason I found out was the monthly statement I got today from uc, it shows my uc payment going up by £120, but also esa deduction going down, so basically im getting the same amount, but esa tell me my esa is not going down and to speak to uc.
I phoned esa and they say my payment is not changing.
However if uc keep overpaying me im going to end up owing them quite a bit