I have run a test on several on my blog pages that I tweet about, and noticed that. Not convinced by that since I don't have that much of followers, I started to analyze my pages with other twitter users.
I noticed the same rounded factor emerge from people retweeting my pages. Here are some of the statistics to totally unique pages
Twitter1: 6800 Followers -> 670 visits
Twitter2: 580 Followers -> 50 visits
Twitter3: 10,200 Followers -> 1011 visits
Twitter4: 220 Followers ->19 visits
Twitter5: 900 Followers -> 88 visits
So If you repeat your tweets, the other 9/10 of followers can get to see it and click it. Of course it depends on the way you tweet too.
Again that doesn't mean this is absolutely true, maybe you guys will come up with different results.