When viewing time spent by hour, the resulting report can display more than 60 minutes. For example, a visitor views a page on your site at 12AM, again at 12:25AM, again at 12:50AM, and again at 1:15AM. In short:
00:00 > 00:25 > 00:50 > 1:15
The time spent metric takes into account all three gaps between image requests, causing the hour breakdown report to show 75 minutes.