To Read and Not Understand?

This is a remnant of studying in university, but I realized I have the ability to read and not actually retain any of it (ok, so this is nothing new, but as those of you who know me well, I tend to repeat stories a lot). I can somehow read whole pages and while my eyes are scanning the words, my mind can be in a totally different place. It's an odd sensation when you really think about it. Five minutes later you wonder where the last five minutes went, and that you're reading the same paragraph for the 7th time.
Maybe it has to do with my short attention span? Hey look! That dog has a puffy tail!