What I would wonder about is how these two measurements were taken. Seems like some of the common techniques may use a 2-D imaging technique, while more modern instruments may use 3-D. If 2-D, what assumptions have they got to make to get to chamber volume, and how reliable could the resulting ejection fraction be?
There is an article on emedicine regarding the effect of beta blocker on ejection fraction. Might not exactly line up with your situation, but may be of interest:
www.medscape.com/viewarticle/562472
On the EF matter, I note 75% is on the high end of "normal", higher is not better once one gets up to 70% (best I can remember), and 49% is just fine, low end of normal. The left ventricular hypertrophy could be the reason for the high EF (75%) not sure what to make by comparison of the 49% reading. I've gotten considerably different readings on the diameter of my left atrial (important for my AFib problem) and my cardiologist has never give an answer for why... all measures were made by the same technician. Unfortunately for me, the enlarged left atrial seems to be the correct reading and thus I continue to suffer from AFib with not much hope to cure it.
I have never been told anything about beta blocker effects of EF... I have an EF of about 60% repeated in several echocardiograms. I think it is common for different methods of measure of EF to give different results - but I don't know anything about EF measurements from stress tests, I've had a few of those too.