This may help your understanding.
Designing websites at present is a right royal pain in the ass. The difficulty designers have is comparability of their site not just with the many browsers but their different versions as well as making them work with mobile devices etc.
Not surprisingly M$ browser was one which would often cause even the simplest of design elements to "misbehave" so the designer would have to check which browser version you are using and write the html specifically for your browser. This can complicate the design and take hours to get some elements to display correctly in all browsers. It would also mean on going time spent tweaking the site to ensure it still worked with future browser updates.
The easy out is for a designer is to limit the browser version their website will work with and then test if your browser meets these requirements. The string returned by these test doesn't always return what you expect. For example testing for IE could return
"Mozilla/4.0"
Which to the designer is enough info but to the rest of us might suggest we are using some weird browser
Hope this is helpful