How are ratings for listed races determined?West Australian Racing

Legless

I have looked however cant seem to find the answer - may be under my nose. apologies if it is.

How are listed races given their rating - why not all set at 90 and what is the theory behind the ratings penalties applied?

BEAUFINE STAKES METRO - 3UP LR (+1) 88+ - Settle Strada wins goes from M81 to M90 ( why not M92? )
H.G. BOLTON SPRINT - METRO - 3UP LR (+2) 86+ - Settle Strada wins goes from M90 to M94 ( 4 points for win )
BELMONT NEWMARKET - METRO - 3UP LR (+2.5) 86+ - Settle Strada wins goes from M94 to M98 ( 4 points for win )
GOODWOOD SPRINT - WFA - Settle Strada wins goes from M98 to M100 ( 2 points for win )
FARNLEY STAKES METRO - 3UP LR 90+ - Kalahaar wins goes from M87 to M93 ( why not M94? ) Settle Strada 2nd ( no change to rating )

BOULDER CUP -Listed Race- METRO - 3UP OPEN 86+ - Fanico wins goes from M84 to M90 - ( I'm guessing 86 + 4 for win )

HANNANS HANDICAP - METRO - 3UP LR (+3.5) 85+

At a guess - if Kalahaar wins gets 4 points goes from M93 to M97
If Rons Call wins - what happens? Goes to 89 ( 85 + 4 for win)

I just can not seem to get a grip on the rating system for listed races - any help explaining greatly appreciated  ~X(