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

OK sorry about the confusion.
So my normal uc payment is about £1500 with a deduction of £600 because I receive esa of £600 per month, meaning my uc payment is £900.
However now in my new statement it gives my uc payment as still £1500 but my esa deduction is £480, meaning my uc payment is £1020.
I rang esa and they say my esa payment has not changed and I will still receive a total of £600 a month ( im paid 2 weekly but uc work it out monthly) from them, that means something is wrong somewhere and I will end up with an overpayment of £120 of uc every month.

No circumstances have changed