Bookies spend a fortune on odds compilers to set the original prices - on stats- after that its almost entirely based on liabilities. Punters follow form or gut. Over the course of a book running (this one is 9 months) the narrative will change.
The bookies ensure by the end, however slight they simply make a profit. They run thousands of 'books' every day. they just make sure they are on the right side of it in as many of those books as possible. They get it wrong all the time, particualrly on short term books, as in 1 game or 1 race. Over the course of longer bets they will rarely lose as they able to offset potential losses many times over as punters pile in on both sides. They don't know, they don't have to. Celtic are a big price based on the form of course, but predominately because the huns have been piling on.