To maintain the distinction between professional administrators and
Mac OS X users with administrator accounts, avoid using the noun
administrator by itself to describe a person who has an administrator
account in Mac OS X. Instead, say an admin user or a user with an
administrator account-typically, the owner (or owners) of a particular
Yuhong's comment was that maybe we need to differentiate the professional administrator from someone who happens to be running as an admin.
I'm not sure that there's any meaningful difference, unfortunately, except for the fact that the professional administrator hopefully has more training (this is not always guaranteed unfortunately).
Except in certain environments (mostly large corporations), the idea of a professional administrator is essentially gone. Back before my mom retired, she ran a number of professional organizations (the last of which was the Society of Women Engineers). All of these organizations had relatively small offices - essentially your typical small business. There were a half a dozen full time employees, and they didn't have the budget to have an administrator. Instead they contracted out with a company to setup and troubleshoot their email servers (which sat in a storage room in the office). They treated the systems as same way they treated the HVAC - the box sat there in the corner doing its thing, and (just like the HVAC system), they called in the professionals when it broke (and paid about the same obscene hourly rate as they would for the HVAC repair). But for the half dozen or so computers in the office, the various people working there managed them themselves.
Raymond's right - these days users are expected to perform all of the tasks that were normally associated with a professional administrator. The professional administrator (or BOFH if you'd rather) of many machines is the 12 year old kid that lives next door (or the person who rides at your barn whose husband happens to have worked at Microsoft for a long time :)).
For consumer operating systems, there is no meaningful distinction between a professional administrator and a user with admin rights.
I believe that the Apple style guide quote is an attempt to make this clear - when you use the word "Administrator", it implies that the comment applies only to professional administrators, and not your riding instructor, when in fact it does apply to her, she just doesn't know it.