Donald Trump - rule and exception

Artwork The Guaridan/Tarek Chemaly

I saw it coming - Donald Trump said that the newly elected mayor of London, Sadiq Khan, would be an exception to the ban he imposed on Muslims. It was obvious, anyone who sets a rule allows him or herself to set exceptions - just like that, on a whim.
Why Sadiq Khan and not other Moslems? Go figure. But I ceased to be amazed by how people contradict themselves. A student was upset at another teacher so he says talking to me "look at him, he is so short!" I gently pointed out that I myself am short. Without batting an eyelash he replied "yes, but you are different" - I don't think the baffled look on my face was enough for him to understand.
Another instance, the HR of the company announces that computers of the company cannot be used for personal purposes. Once more, I tell her that my gmail page is always open as soon as I come to the office. "But this rule I made up for the girls downstairs, not for you." Again, a rule, an exception, no justification.
And the list goes on.
Is it inbuilt in the notion of power to be able to toy around with the rules and selectively apply them? For all my shortcomings, I try to be egalitarian among my students. No eating or drinking in class applies to everyone - even that one who did not have time to eat anything in between classes. No whatsapp mongering gets everyone - and I mean everyone - chided and ridiculed "you are not a brain surgeon and your patient is not waiting for you in the operation room, this can wait".
Do I have preferences among my students? Sure! But I restrain myself and never give a hint as to whom I have fondness for, even disguising this as irony for them not to even know (I am most ironic with those I like lest they know they are preferred on a personal level - because, it turns out I am human too).
Still, the question of rule and in-built exception, or the notion of "yes, but not you" (of which I have been a recipient for unknown reasons) still baffles me.
When I was in a ministry for an official business and I was barely 18, an army soldier singled me from all the waiting file, took me by hand, got me my needed signature in 2 minutes, and I still have no idea why he did it even if I was obediently waiting in line.
"Yes, but not you".
Sometimes I am lucky I guess, even if I make everything possible to defer that luck.