No two engines will ever have the same performance. Not even two new production engines in serial number sequence will have the same performance. Most of the time, the differences in performance between engines is fairly slight so there is not much if any change in throttle position. The bigger the engine like a JT9D, the more likely it will show throttle stagger than it will on a small engine like the BR715. In addition to the items previously listed, another cause for throttle stagger is engine deterioration. As the engines age, the throttles have to pushed higher to attain a given amount of thrust. With engines of similar vintage installed, the added throttle push is not noticeable because the engines will most likely age the same and the throttles will be pushed forward similar amounts. If one of the older engines is replaced, the throttle for the newer engine will not have to be pushed as high as that on the older engine to attain the given amount of thrust, hence the throttles become staggered and difference becomes obvious. Engine deterioration can be many things: worn compressor and turbine outer air seals, bowed or burned turbine nozzles, dirty compressors. As was previously stated, there are limits on how far the throttles can be staggered. Most allow up ot one knob width before maintenance has to adjust the throttle rigging or engine trim.