Too Many Five Star Reviews Out There?

Is it just me, or are there a disproportionately high number of five star book reviews out there?

I try (and fail) to review the bulk of what I read, even if it is just a one-liner, but often feel more than a little sheepish submitting my four stars next to a rash of gushing five stars, my three stars next to the fours. Or am I just an tough reviewer?