Probably not of any help but my understanding was the same as yours i.e. if you turn on automatic downloads, use iTunes match or re-download past purchases then that computer or device becomes associated with that account for 90 days : http://support.apple.com/kb/HT4627
But I've seen other people post the same experience as you have, so it's not clear to me when it does or doesn't start an association. When people ask how to get content that their partner/spouse has bought onto their device I usually suggest that they copy it from computer to computer (e.g. via a flash drive) and add it to their iTunes and sync it as opposed to re-downloading it due to the potential 90 day association.