I think a minor point is being missed with the HEAVY, SLOW scenario.

Let's say the meeting is Saturday and Friday it's a bright sunny day, we are all doing the form. Track Condition: Good

Let's say by Friday night, the fav is horse 1 paying 1.50 and we are ready to back it to the hilt. ( it's a true fav by the way)

Friday 11.30 PM, the weather changes and there is a sudden down pour of rain, by 6.00 AM Saturday, Track Condition : Slow/Heavy.

Trainer scratches the 1.50 favourite.
New "favourite" is horse 2
It's price is 2.50.

The question is,
does the database (anyone's) take into account the scratched true fav. from Friday's form, or is the "fav" that whatever it is on the day regardless if the first fav. was scratched ?