another reason, what I have noticed. you often get asked to sign in to wifi with an e-mail address. My wife did so once with her own e-mail address and as a result she was enrolled in WOH.
the pure number of members is actually totally meaningless.

The member number in AAdvantage will drop drastically when accounts are being closed in the next couple of weeks/months. Maybe the account from my dad who passed away in 1996 will be deleted as well. It was still in the system in 2010.