An EKG is only a tool to help diagnose a medical problem (usually electrical
impulse integrity) and not very good for chamber dimensions. The left atrial enlargement will require an echocardiogram to validate. If significant it could indicate high blood pressure or mitral valve regurgitation (back flow of blood) or opening stenosis (restricted size of opening). The mitral valve is a one-way passing of blood from the left atrium to the lower ventricle to be pumped into circulation. When the heart pumps there are leaflets that should close over the opening. Often the leaflets don't tightly close...not uncommon.
Sometimes either the right or the left bundle branch (pathway for electrical impulse) conducts the electrical impulse more slowly than normal, but is not completely blocked. When this occurs, the impulse may be slowed and the electrical impulse arrives in the ventricle slightly later than normal. This can cause an irregular heartbeat... dropped beat or two fast beats are not uncommon and not considered a medical problem. Medication, diet and exercise can have any influence of the EKG output. Also, there can be artifacts that are medically meaningless, and due to the equipment and/or placement of the EKG leads, poor conduction, etc.
Normal sinus rhythm indicates the heart's pacemaker is providing a normal heart rate and not picking up any impulses from another source that is causing an irregular heartbeat. However, the heart's pacemaker is or has caused an irregular heartbeat during the test. It could be the heart rate was over the normal resting rate of 60 to 100 during the test, and the underlying cause was some anxiety, etc. The doctor will make decisions based on your health hisory, other tests, symptoms etc.
Hope this helps, and if you have any further questions you are welcome to respond.