No, the icons just indicate releases. They're at different vertical levels so they don't obscure each other. :-)
I decided to start in 2006 to catch Firefox 2 and Internet Explorer 7, otherwise you don't fully see the gaping hole which represents the Dark Ages of the modern web. (Safari was added later, and 2.0 happened to slot in at the start of 2006.)
It would be pretty cool if you extended it back to 2001, when IE6 was released - then you could also show the rise and fall of Mozilla and the birth of Firefox.
Even better, roll it back to 1991!
Edit: and another pony request - how about putting the release date in the tooltips (yyyy-mm-dd) to clarify any confusion about when exactly it was?
I decided to start in 2006 to catch Firefox 2 and Internet Explorer 7, otherwise you don't fully see the gaping hole which represents the Dark Ages of the modern web. (Safari was added later, and 2.0 happened to slot in at the start of 2006.)