Haverford Co: Different treatment of Contract asset/liability on IFRS 15 Revenue

1, I have watched this example of Haverford March/June 2018 of OpenTuition:
https://youtu.be/t5EJFE1-qmk?si=CdFWdhhd-EJqJTAx.

But BPP has different treatment, they solved Contract asset as Revenue recognised less amounts billed = 5600 – 1400 = 4200 not 1900 + 2000 – 1400 = 2500 as OpenTuition and ACCA. Why do we have this different treatment. In addition, textbook of BPP wrote Contract asset = Revenue – amounts billed. Why are they different, what to bring to the exam?

2, Moreover, what are the entries for including Revenue and COGS to adjust for Draft profit or Draft PnL. Some situations, they booked Dr COGS \ Cr Inventory and Dr Receivables \ Cr Revenue. Whilst sometimes people booked Dr COGS \ Cr Customer contract and Dr Customer contract \ Cr Revenue.

I believe that this is a mutual concern. Have a great day, thank you so much!