its because its better to have the rudder on the left then on the right so it would turn better unlike mono hulls have trim tabs and turn fins on both sides.
the rudder is a personal issue, some people put it on the left, because they like the pull effect from the servo, i like it on the right because it move away from the prop when turning.
My preference is also on the right side. That gives both of the turning pivots - turn fin and the rudder, on the same side. Just found the tracking easier and the turning arcs more predictable in the turns. Since I run only riggers, that is a must for me.
If the rudder is mounted on the left side of the transom?? it is possible it is an older kit?? the builder was looking for Cross Weight. he was tring to transfer weight to the right front sponson when the boat turned right & keep the turn fin hooked up. Today we have Bigger More powerful Digital servos. Most current mfger`s mount Rudder on Rightside today & let the servo do its work. Joe W.