Just leave it to doctors to make things as confusing as possible!!
Both your father and your uncle had heart attacks. Myocardial infarction
is sorta generic for heart attack. Loosely translated, it means lack of oxygen
(or lack of blood flow) to the heart muscle.
There are lots of things that can cause a lack of oxygen reaching the heart muscle. The most commonly understood reason is plaque in the artery
that blocks the blood flow to a particular area of the heart muscle. This is what your father had. It's associated with the most common things you hear about heart disease: high cholesterol
, family risk factors, bypass surgery
A heart attack can also be caused by a "free flowing" blood clot that wedges in a coronary artery, blocking it off. Some dangerous heart rhythms can cause the heart to not receive enough oxygen, spasms in the heart muscle and a "weak area" in a coronary artery that bursts (anuresm) can all be called heart attacks.
Why so specific on one and not the other?? Sometimes a doctor doesn't know "exactly" what caused the heart attack. If they do know, some doctors are really particular about being very specific like your father's doctor (that is really, really specific)and some aren't. If the person signing the death certificate isn't the patient's regular doctor they are often less specific, unless it's a pathologist which is usually very, very specific (due to autopsy).
I hope that clears things up a little bit, and gives you some understanding. I'm sorry for your losses.