A good explanation IMHO is that mibbit is an app with a very techie audience and these people tend to use non-IE browsers and non-Windows OSes.
I like your explanation of valuing time and I know a lot of techies that use OSX as "unix without the hassle". A lot of them are web devs, which again fits with the mibbit having techie audience again.
Did you check for correlation between Safari and Mac? That is, is the fact that Safari scores tops because people use Macs? What is the revenue of a Safari/Windows and a Safari/OSX user?