Transitioning to UC - Tax Credit Overpayment?

[BabyCloud]

Hi.
I transitioned to UC the day after I had received my weekly payment of child tax credit - I used to get around £75 per week.

This morning I received a letter stating they have over paid me by around £93. Is this correct? I guess this was due to the final payment and the day I applied. Did I make a mistake moving over the day after their payment?
It baffles me that we can move over within deadline but still be classed as overpaid.

Does the letter explain how they've arrived at £93?

What should happen is that when you claim UC the Tax Credits claim is closed effective the day before you claimed UC. Tax Credits will then finalise your claim based on your actual income up to the date the claim closed. It's not uncommon for that process to identify either an under or overpayment. Is it possible you earned a bit more in the current tax year compared to the one on which your tax credits were claculated?

The same thing has happened to me we switched in November and yesterday got a letter saying I owe £450 to tax credits.

The exact same thing has just happened to me - Also £93.

I thought it was unfair as I migrated over within the time frame they gave me.