When looking at a reader’s profile from the administrative side, you can see both a “Reading Log” in the lefthand navigation menu and a “View Challenge Log” link under the name of each current challenge.
The “Reading Log” section of their reader profile allows you to see all reading logged over all time for that reader. It includes reading that both pre-dates and post-dates your challenges. The “View Challenge Log” link under a specific challenge only shows reading logged with that challenge’s log type and within that challenge’s dates.