Arbusers

When i have arb i get to familiar with standard lines and sometimes i feel that lines are not correct, but still how these companies create lines and offer info to the bookies.? For example nba under/over .Probably they use historic information. But how far they can go?They can analyse last season, but they cant go far behind. 2017 chicago bulls isn't same as 2000 bulls-teams are totally different. Same to H2H match ups. Or other thing. We dont know is Curry injured or not. How could it affect over/under?
Could anyone can give some insight how lines are created?