Nearly any site using a web analytics package will collect browser statistics, even to the version number. Even better if you aggregate the statistics from a number of sites with different user demographics. For example, just the Apple site or the Mozilla site would give you skewed statistics. Most analytics packages will find unique users over a specified time period. Given that you use all your browsers at the participating sites, you would be counted as a unique user for all 5 browsers, distorting the statics somewhere off in the 6th or 7th decimal place.
Most of the estimation is done by log analysis, hitlinks.com is one such party, they have a panel of several 10's of thousands of sites that they get browser information for.
If you have a large enough portfolio of sites then you can do your own analysis.
It doesn't look like you are a 'typical' user, who will use a single browser only unless they run into some kind of compatibility problem.
Do you use the different browsers to test functionality or because different sites you visit function better with each browser? Just curious, as I pretty much only use FF3, but I will test my sites on FF3, Safari, Chrome, Opera, IE6 and IE7...in which case I would assume they wouldn't get counted as I'm doing internal testing.
On an average day, I'll use Firefox 3, Chrome, IE7 and sometimes Konqueror & IE8. How would that get counted?