Like I said, it's not unusual for the mounts to be a little off, they can be spread apart and squeezed together because there is some flex there. Usually though it's the outer tabs that have the most flex, not the inners that are attached to the shock mount. And that's what would have to move on yours.
Have to measured the width of the arms? Are they both even the same width? I don't know where those UCA's are made, and manufacturing defects do happen.
