Is Left Ventricular Ejection Fraction (LVEF) reliable as measured by an echo?

I have a 50 year old patient who had an inferior wall MI 2 and a half years back and is on Aspirin, Clopidogrel and Statins. He does not have hypertension or diabetes. A year after the MI, his cardiologist reported his LVEF to be 55-60% after an echo. A year and a half later, a repeat echo was performed (for routine purposes), and the cardiologist reported 50-55% LVEF and Grade 1 diastolic dysfunction. The patient is currently asymptomatic and is here for regular follow up. However he is a smoker but smokes only 1-2 cigarettes per day - he is on a plan to quit smoking gradually. I was wondering whether the decrease in LVEF is worrisome? Is LVEF a reliable measurement? Is it true that it undergoes day to day variations? Any help would be appreciated! Thanks!