Why are you surprised?
If you have folders set up as
Then even by the rules of Windows paths, the way to get to /css from "main.aspx" is via "./css/master.css" where as from "another.aspx" it would be "../css/master.css"
So you either need to specify the correct *relative* path from your "another.aspx" file or you must specify an *absolute* path that is relative to the root of your site.
If your /root directory (or whatever it is named, but corresponding to the above picture) is also your web root directory, then you can always get to "master.css" from *ANY* page by using "/css/master.css". That leading "/" means "start at the root directory".
There are other solutions, especially if you control setting up virtual directories on the server, but that's the meat of it.