An explanation tells you the cause of something. An excuse tells you why a person had no other choice except to act as he or she did.
For example: “I treated you badly because I was drunk.” Being drunk explains the lack of rationality; but it doesn’t excuse the choice to become drunk. Or: “I am nasty to my husband because my parents were nasty to me.” That explains where the nastiness was learned; but it doesn’t excuse the choice not to learn new habits and attitudes.
People who merge excuses and explanations into one are using psychology, science and reason to evade (or rationalize away) the reality of choice. Sometimes we have no choice, but most of the time we do. It’s helpful to find explanations for things so that we can make better choices in the future–not so we can ignore that choices exist.