Are you sure the payout was exactly $36.60? If so, it's not what we would have expected, but there are some possible reasons for it.
Here's how we figure it.
If you bet the pointspread, that's calculated on the basis of a bet-11-to-win-10 equation. In order to have earned an exact $20 profit, you would've had to bet $22. The payout on this would then be $42. This is the standard sports-betting odds, often written as -110. It's how the bookies get their edge. When you lose, you lose a little more than you win when you win.
Since you only bet $20 even, the house percentage (also known as the "edge" or the "vig") was subtracted from the payout. Divide $20 by 1.1 to get $18.19. Had your bet been at a normal -110, you should have been paid $38.19 or something close to that as sportsbooks do round off cents sometimes.
If you're certain that you were paid $36.60, a possible explanation is that sports books don't always use -110 as the odds on every bet. Sometimes, rather than vary the point spread, they vary the moneyline associated with that pointspread.
Normal odds on a matchup:
Team A +5 -110 Team B -5 -110
This is where you're laying 11 to win 10 on either team.
Less commonly, you'll see this:
Team A +5 -120 Team B -5 +100
In this case, the pointspread of five is the same as the example above, but Team A is more expensive. On Team A you have to lay 12 to win 10, while Team B will pay even money. The edge is the same, since the house is still charging 20-cents juice (in the first example, it was 10 + 10, while in the second, it was 20 + 0).
It's possible that your bet was represented by example 2 and was at -120. If that were the case, then take $20 and divide by 1.2 for $16.67, which should have paid $36.67. It's possible that a book rounded down to $36.60 and that's what you were paid.
Note that whenever you purchase a sports ticket, the team name, the odds, and the payout are all listed on the ticket. When you buy it, stay at the counter and verify the ticket. Make sure it says what you wanted to bet. Check the payout amount at the bottom. If it doesn't agree with what you think it should be, ask right there. Usually, as long as you don't leave the counter, the sports books will be willing to make a change if there's a problem or disagreement. Once you leave the counter, it becomes more difficult. Check those tickets!
