My undertanding is betfair sp is calculated by settling all sp bets + exchange bets (which are set to go live or sp).

Its not like the tote where they just take 15%. (in this case 5%). So the sp can fluctuate quite a bit.

The issue is the liquidity in the market. While there may be 10K in the exchange betting up till the market is suspended, only a small percentage of this may change to sp. So where a horse had $100 at $4 $300 at $4.20 and $500 at $4.40. When the market gets suspended the $500 + $300 amounts might decided to lapse rather then converting to sp. So there is now $100 at $4 that is available. They add his $100 at $4 to the betfair SP then they try to match up the available SP bets.

Its highly open to manipulation by large money (intentional or not) due to the low amount of dollars going through it.

Does that make sense ? Betfair has a full page on it but I recon you already have had a read. It does give and example I think.

Any questions just ask.