cbESA and irESA

For over 10 years I believed I was on irESA and filed my P60u's. I have completed the move to UC and find out what I will get in a few days time. A few days ago I received 3 letters from the DWP. One was telling my my irESA is now finished. Another letter with a P45 and a third letter telling my how much I had been paid in cbESA up until my last payment (the one you get two weeks after a UC claim is made). This letter mentions the 2025/2026 period but does not state when from. I presume it starts at the beginning of the tax year. I calculated how much cbESA I was getting over 2 weeks. The amount is around £20 more than what I am actually getting in total. It is so confusing. My question is do the DWP work tax years from 6th April or do they calculate it from 1st January. My P60u runs 6th April until 5th April the following year. Thanks for any help with this.

Thanks David.