Well again (if) they say 10-15, then it's really 10 and that's under optimal (lab) conditions. I guess at minimum the following would apply to reach that number:
1. You have a pristine cell signal always (ie: you are standing under the tower and never moving).
2. Since you are making calls your screen is never on and in the brief moments it is for calling or answering, screen brightness is set to minimum.
3. You are not using any mobile data or wifi for anything.
4. All background services (sync, location, etc.) are disabled.
Try it again under those conditions and see what you get...