Apple can use private/undocumented APIs which are forbidden from App Store apps. I believe you can run a strings command on the itunes file to see what the APIs are called.
This is complete speculation, and not even good speculation.
Gmail for iOS is essentially a single-site web browser, whereas the Mail app is a full iOS app utilizing the full CocoaTouch frameworks. A native app is going to have a faster experience.
Also, use objdump or something, like a professional -- not strings.