You may find it improves with use as it squares up more, it only takes a little muck on one side to cause imbalance.
Also are you turning both wheels in the normal direction of rotation? (as though car was going forward)
Reason being brake shoes are designed to work more efficiently in that direction hence leading and trailing shoes are different.
In the old days where two wheel cylinders were used on front brakes for instance , both shoes were similar leading shoes, but where a single double acting cylinder is used, such as your rear brakes then two different design of brake shoes is used a leading and a trailing shoe.
Note this is probably not worth you thinking too much about as all you want is an MOT.