Per Dave's explanation....Let me add that for a nonadjustable stab, the right elevator will require up trim and the left an equal amount of down trim. It shouldn't take much more than 1-1 1/2 turns at the clevis. There have been some other but less effective measures discussed over the years also, such as raising both ailerons slightly. Also larger stabs tend to be more prone to this flight behavior than smaller stabs.....

Think of what's happening with top left rudder; the right stab half is hanging below in relative cleaner air which makes this half's trim more effective or more dominant.

This type of flight behavior can often occur when a model is nearing very good trim.