Its not hard to understand. They take the market just at SP and apply it to the exchange prices at the jump of the race. It includes exchange money not just SP money. The SP money could also have limits and not be included.

Read this and see if it explains it to you:

https://www.betfair.com.au/hub/gett...ing-price-faqs/

Go through each question even if you don't think its useful as they hide some facts in there. For example the best explanation of how SP actually works is in the last question on "What is the near price and how is it reconciled".

That basically gives you the SP calculation but its run now instead of at the start of a race.

If you have more questions ask again.