Thickness from side to side, be it inboard/outboard on a single rotor, or on an axle, do not need to be exactly the same to each other.
Unevenly worn rotors, inboard/outboard wise, will happen with a single piston caliper. A nominal difference, inboard/outboard and axle, isn't anything to really freak out over. Drastically unevenly worn rotors may be a sign of other trouble, though.