Have a look at the scrum guide and you will see that the team is expected to spent around 10% of their time reviewing the backlog. With all the will in the work, this is not easy to achieve. Even a highly disciplined team will find this difficult, especially when trying to spread the time across the iteration. Sure it is a little easier near the tail end of the iteration, but what this means is that there is a lot of work on the backlog very close to the start of the new iteration.
