Its based purely on price/odds. He may apply any of the factors you mention in determining that the price is wrong, for example, a drop in trip. If he used the same selection criteria everytime for making a selection then he wouldn’t always get the best valued priced selection.
Dare I say (and no offence to Gingertipster) that he uses Gingers value concept but generally makes just one selection in a race rather than 2 or 3 