back payments

Hi there, I'm writing to you before I ring them to make sure I don't make a fool of myself, to ask you if you're awarded the enhanced rates for both care and mobility, which I have been, thanks to all the help and advice you give (BEST £20 I'VE EVER SPENT), if one of the rates has increased from when I was receiving DLA - I was getting the middle rate for care and now the enhanced rate in PIP, an increase of £28.95 a week, how many weeks do the dwp back pay you. Is it from the date you return the claim form, the date you're assessed or when they make their decision. I'm asking you this question because I can't figure out what it reads at the bottom of the page which shows what I've been awarded. It reads -

"We will pay £262.95 into your usual account on the 9th October 2019. This is money that we owe you from 18th September 2019 to 8th October 2019.
We will pay your benefit into your usual account every four weeks in arrears on a Wednesday."

The figure they give is 3 weeks of the enhanced rate for care. But what about all the time before the 18th September when my disabilities were no different to when I sent in the claim form and when I had the face to face assessment.
My claim form had to be returned by the 15th May, my assessment date was on the 5th July and the date on the letter which confirms my awards is the 19th August.