I have a theory:

All people of equal ability are of eligible to equal reimbursement for their labour, therefore any salary difference between you and your friends is the opportunity cost of your life decisions.

I’ll go on and make all my disclaimers after the fold.

So, that all seems a bit mean, but let’s see how we go here.
The first objection might be about how to measure “equal ability”: this seems to be a tricky one! But I think it can be solved reasonably easily. Two people would be equal if they had no comparative advantage over each other. This seems a bit too obvious, but in reality you probably have a feeling for a pecking order of capability amongst the people you know. If someone lower down the rank is getting more cash than you at the end of the month then you have either ranked them wrongly or they are paying in some other way (eg they work crazier hours than you, or they have to deal with some real arse holes every day).
The other objection is that this is so staggeringly obvious that it isn’t worth saying. Well this might be so, but everyone who I’ve mentioned it to seems to think it novel and entertaining enough to make them giggle a bit. I’ve always put it in slightly different (possibly less accurate) terms; something like “the difference between your salary and that of the guy at uni who you were smarter than is the cash value of your job satisfaction”.
The other thing that springs to mind is that it ignores the role of luck, but I don’t have much time for luck, you can steer it and make ‘lucky breaks’ more likely. That conference where you met that person who, a few years later turned out to be really useful… That happens all the time and is a life decision.
The job satisfaction point I make above is a bit too coarse to catch the full impact of this, but it does go some way to illiterating it, and a lot of things can be boiled down to job satisfaction I suppose!
I’ve started reading The Wealth of Nations and _Case, Fair & Oster _so that might be coming through here.
I’d be interestes to see if anyone can come up with a knock down argument against this, or a better wording to strengthen it (comments please).