O/U.. When one side is juiced?

What causes, for example, the under to be -120 and the over to be +105?

It's almost like Vegas is begging you to take the over in that example, but as I track them they seem to hit at the same roughly 50% rate. 

Thanks for any thoughts!

@Dutch1976

 The book wants the bets to be balanced.  If they get even bets on each side at -110 each then they've just made 10 percent of every dollar.  They will adjust as the money starts to become unbalanced.  So the line above suggests that there is more money coming in on the under than the over.  So they are willing to pay out more on the over to bring the money back into balance.

That's just the sportsbook juice in effect there! The under in that situation would be slightly favored but not enough to move it 1/2 or full point

@SomethinCr8ive

@bettor22

 Both good points... thanks