Here's my take on it.
The goal of the casino / line makers, whatever you want to call them, is to make the most money off the public.
Let's say a total of 10 people. 5 people bet on the Patriots, 5 people bet on the Giants, both ATS bets, each person risking $100 to win back $191. Total money pot is $1000.
Ok, now let's say the Patriots win. 5 x $191 is $955. $955 goes back the the public, the $45 goes to casino, the casino makes a profit of $45 off the public.
If 4 people bet on the Giants, 6 people bet on the Patriots. The public wins.
If 5 people bet on the Giants, 5 people bet on the Patriots, The casino wins.
If 6 people bet on the Giants, 4 people bet on the Patriots, The casino wins.
The fact that when you risk $100, you are not doubling your money to $200. It's usually $191 on either ATS side, this gives the casino the house edge. Depending on how the public is dumping their money, the value can go above $200 in the publics favor.
There's line movement, but there's also value movement.
Also, watching this video could make you smarter: