Onions:
The odds posted for a race are win odds ONLY. While you can calculate "fair" payoffs for daily doubles, exactas, trifectas, etc. based on win odds, the payouts themselves have nothing to do with the win odds. pd1 correctly pointed out that it has to do with money in those separate pools. To calculate an exacta payout for example, you would take the total $ in the pool, less the enormous track takeout, divided by the number of winning tickets. If the exacta pool is $1,000,000 and the track take is 19%, you are left with $810,000 in the pool. If there are 9000 winning $2 tickets, then the payout would be $90 per $2 ticket.
There oftern overlays (and underlays) in exotics pools and these make for some good betting opportunities. This also shows why exotics payouts in general, and trifectas in particular, are much higher when the favorite runs out of the money. The favorite usually has a lot of $ bet.