DWP Payments Moving From DLA To PIP

First and foremost a huge thank you for the guides. I have just moved from DLA to PIP with the same enhanced mobility and care. It was hugely distressing coming during my father being taken ill and then dying but they agreed to a 2 week delay for sending in my forms (big deal!) This is my 3rd success with using the guides and I know I could not do it without them, so thank you! x
Now to my question. On my letter confirming my award it was dated 27th Nov. It said they would start paying PIP from 25th December.
I had a normal DLA payment on 17th Dec which was 4 weeks as usual after the last one. If DLA is paid 4 weeks in arrears, that means that I would have been paid up to 19th November. So far so good!
I was then paid a DLA payment of one week of (£87.65) on 24th December, which would then take me to 26th November for DLA. The letter dated 27th November, says they will pay me £325.56 on the 20th January which they say is money they owe me from 25th Dec to 19th January.
So if PIPis paid 4 weeks in arrears like DLA, I dont know what the £325.56 is representing? 4 weeks from the 19th January takes you back to 22nd Dec and I assume they have deducted two days because they think I got paid on 24th December BUT.... as I said above that payment was till the 17th December. So I seem to be missing a)17th - 22nd December and b) why have they DEDUCTED the equivalent of two days when it should have been for a full 4 weeks?
The letter makes no sense at all when it says they are paying me from the 25th Dec... why?
Sorry this is so confusing its taken me all afternoon and printing out three months calendars to work this out! If anyone can throw some light on this Id be grateful. Thank you.