I don't really know for sure, but I'm guessing the reason is due to the summer. I suspect that the majority of web hosts' business in the shared hosting market comes from the home enthusiast. During Winter, and the long Winter nights, people are more likely to spend long times online, as it beats going out in the cold. Whereas in summer, people are more likely to stay out late, in the Sun, instead of staring at a computer screen.
Just a theory. I know it works with me (my online costs are greater in Winter than Summer).