Both, but in a different way and for different reasons.

When the bookies very first put the odds out they may have mispriced the market, and since they have to offer odds on such a huge range constantly they do slip up sometimes, in effect they are forced to bet on every match. You do not have to bet on every match, or even every day or every week, so you can just cherry pick the value bets and wait for the betfair market to form later to lay them off. I say "just", you do need to be able to make a book on each match and have some idea of what the odds should be to spot the errors.

And also the best odds are just before kick off as thats when the most money is put down, last minute news is take into account and the odds fluctuate the most, making close matches and arbs at betfair possible.