The glaringly obvious part is interpreting word-for-word the documentation.
iTunes Store: Associating a device or computer to your Apple ID - http://support.apple.com/kb/HT4627 - In connection with, "When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID." "Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days."Re: iTunes account Query
As the above indicate and say, the 90 day limit applies when you wish to change authorization for the purpose of re-downloading (make sure you see the "re" as in download a second time, not the first time) or use iTunes Match, or automatic download to ensure all purchases are automatically on a computer (all basically forms of re-downloading music). You can use your computer for multiple accounts and multiple authorizations. You can each make and download new purchases when you want. It helps to make sure you have all purchases backed up so you do not need to frequently re-download, that's when the problems arise.
There is statement as to why this is in effect, but you can see it would deter media piracy by preventing groups of people going around to friends' computers and having media download parties.