Mac OS X has about 7% of the market share now .
http://marketshare.hitslink.com/report.aspx?qprid=8 thats a fair difference from 2%.
OS X is a document and window orientated OS rather than just an application orientated OS. so if an app allows you to open multiple windows, clicking the "X" button will not quit the app but rather close the window assuming you have another window open or you still want the app to be in RAM. examples of this are Finder (which is always open), Safari, Mail, iTunes, iCal, Terminal, Photoshop & Pages documents etc.
but if an application only has one window clicking the "X" button will quit it as you will have no other use for the app when its only window closes and these apps usually open fast anyway. examples of this are System Preferences, Disk Utility, iPhoto, Calculator etc.
for Windows we all know its different. when you click the "X" the application usually closes. but explorer which like Finder is always open and you assume it quits when you close a window but its always open, its just not obvious. but even some Windows applications use OS X's idea by closing to the system tray. its obviously seen as an advantage of having an app open without a window open of having it in the taskbar. the taskbar is like a crippled dock. then theres applications in Windows that dont close if you have multiple windows open just like OS X does. example of this is browser windows and Photoshop documents. you dont want Photoshop closing after you close a document
what if you want to open another?
the Windows way is too untidy i think with some apps quitting, some closing to the system tray and some not quitting as you have multiple windows open. this is why i think OS X's way is better. its more efficient, user friendly (if you learn to understand it and not assume it should work like in Windows) and it makes sense.
case and thread closed i reckon.