Sure, the odds drift with heavy betting, but it has to be a substantial amount of money for the odds to move more than a tad. And if one bookie is feeling particularly brave in any given situation, he might elect to offer longer odds on a near-favourite, to increase action, and his potential reward. Of course it increases risk too but that's the prerogative of each bookie.
Bookies do rate drivers (Ok driver and car). With numbers, clear as day. Then odds can drift due to market forces. They calculate the odds to create as much action as possible, with a margin at every opportunity. This means offering as much value as they can whilst making money. Their initial rating is unbiased, generally based on a number of expert opinions, and generally turns out to be fairly accurate, given that they are still in business!