Mine have always been different.
With my first, Righty always did a full ounce better than Lefty, until I had an episode of mastitis in Righty, after which Lefty always produced an ounce more.
With my second pregnancy, I think Lefty continued to always outproduce Righty.
This is very normal and a frequently asked question. I guess just like both of our feet aren't always the same, and some ppl have 2 different eyes, our boobs are not identically made.