You would be surprised at how much "smart" money is bet off the track. In the early 1980s I used to make a good living by going to the races midweek and betting the "overs" on the tote compared to the bookies. You could regularly get $10 on the tote about horses that were starting at 9/2 with the bookies. Not any more!
These days there are more and more professionals betting off course so the bookies fixed odds are not as good a guide as they used to be 20 years ago.