The problem is in your assumption. The only time you’ll see an equal amount of betting is if both teams are -105. The +165 means that only about 30-40% of the players are betting on Seattle.

Let’s say that $8,000 is bet on Seattle to win, and $12,000 is bet on New York. That’s $20,000 in action. The house will take 5% off the top, leaving $19,000 to pay to the winners.

If Seattle wins, then you have $8,000 in winning tickets who will get their $8,000 back, plus they share $11,000 from the losers who bet on the Yankees. If you bet $100, you get the $100 back, plus $140 from the pool.

But if the Yankees win, then the winners get their $12,000 back, plus share $7,000 from the losers who bet on Seattle. To get a $100 share from the $7,000 loser’s pool, you’d have to bet $170 on the Yankees.

The shorthand for that proposition is:
Seattle +140
Yankees -170

The thing is that everyone does not walk up and either bet to win $100 or lose $100.

In this scenario, the books feel that a proper line would be New York -170. Regardless of which side you take, you are slightly overpaying theoretically. There’s some interesting sports articles at that you might like. I particularly liked the one they wrote about the run line.

The books change the odds in a way that they awlays win, if people are betting more on one team the price for the other gets up and etc… They win whatever happens.
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