my impression is that the first leg of the knockout rounds always favours the unders, the home teams have to be extra careful as a goal accepted at home will cost them a double disadvantage, and they must carefully try to score too, and the away teams, from the best to the worst would all be content with a goal-less draw, therefore their number one priority is their defensive duties, of course an away goal scored would be welcome if it comes their way, but they can't risk too much for it. But, like said, i haven't yet looked at the figures and past results to verify my impression, and no doubt if there is a bias towards the unders the books should be well aware of it too.
