Why use asymmetrical capacitor values for crystal?
The real question is why would you think that the two capacitors need to be the same, which implies that they serve the same purpose?  Often we do see them the same.  That's the coincidence you should be asking about.

There seeems to be a lot of religious beliefs and so-called "rules" out there for crystal  capacitors, but none of those are substitutes for actually understanding the issues.  The two capacitors serve different purposes, and you also have to consider other impedances in the circuit, not just those of the capacitors.  I have discussed these issue a number of times here already.  See:

https://electronics.stackexchange.com/a/17894/4512
https://electronics.stackexchange.com/a/16025/4512