muga vs echo results

I previously have had a PVC ablation that resulted in correcting my EF, going from 35% to 60% according to echo tests.  Recently I went off the Coreg that I had been kept on for 2 years, and an echo showed it slipping back to 45%, and my doctor wanted a MUGA before deciding what to do.  The MUGA showed an EF of 78%!  I've heard the difference might be 10-15% between the tests, not 75%.  My Doctor is going with the MUGA results for my actual status.  How can the huge difference be explained and should I be concerned about any and all echo tests.  Obviously I'm excited about the 78% EF number.