It was, as always, a matter of protocol. Of discretion. Of careful etiquette. Of, ultimately, alcohol. Or at least the illusion of alcohol.
Lord Vetinari, as supreme ruler of Ankh-Morpork, could in theory summon the Archchancellor of Unseen University to his presence and, indeed, have him executed if he failed to obey.
On the other hand Mustrum Ridcully, as head of the college of wizards, had made it clear in polite but firm ways that he could turn him into a small amphibian and, indeed, start jumping around the room on a pogo stick.
Alcohol bridged the diplomatic gap nicely. Sometimes Lord Vetinari invited the Archchancellor to the palace for a convivial drink. And of course the Archchancellor went, because it would be bad manners not to. And everyone understood the position, and everyone was on their best behaviour, and thus civil unrest and slime on the carpet were averted.
''First we got the bomb, and that was good
'Cause we love peace and motherhood
Then Russia got the bomb, but that's okay
'Cause the balance of power's maintained that way...
-Tom Lehrer, "Who's Next"