Most web sites collect stats on usage. One of these figures is browser type and version.
I've just been using H&R Block for my tax returns this year and find that their site won't work well with anything other than Internet Explorer (IE). Whenever I get an email from them saying that there's a message I fire up IE to go and check the message.
Now the problem with this is that their stats will show that most of their users are using IE. This is because anybody using them will be forced to use IE. So they will never know which browser their users usually use and thereby creating skewed results for their site usage.