It's all about the BES/BIS. That's a server that sit's in the middle of your email host and your device. It constantly queries your email host, something your phone itself cannot do for many reasons. When it has an email from your email host, it sends out the email to your device, hence "push email." Other devices don't connect to an in between server, they connect directly to your email host every 10 mins or 30 mins or whatever time you have setup. So it "checks" your email and "pulls" it to the device.
I don't know how Google accomplishes this w/ Gmail accounts but I really would become an instant Android/Google apologist if they were able to do this with other email accounts.