I am not sure as too why linemakers make NFL lines on a whole number, which enable ties to occur?
I always thought they would want a settled wager and never a push.
Pushes can't make them money? With so few NFL games to make money on why would chance a tie?
I know that some European sites will always adjust to 0.5 number to never have pushes.

The only reason I can think is that say 7.5 vs a 7.0 would just not set the money flow at 50/50, but may set up a big risk for a loss on the game.