ITunes Store account confusion on iPhone

An Apple support document clearly states that when you sync an iPhone, it gets associated with the same iTunes Store account as the one your copy of iTunes is logged into.

"When you sync your iPad, iPhone, or iPod touch with your computer, the account last used to access the iTunes Store from the computer is synced to the mobile device for use in the iTunes Store from the device as well. "

http://support.apple.com/kb/HT1311?viewlocale=en_US&locale=en_US

However, this is not true. I'm sitting here right now with my iPhone plugged into my computer, iTunes showing one account (the one I actually use) and the phone demanding a “Apple ID password" for a different account.

How can I fix this?