The most common reason some people mistake Narcissism for high self esteem is because the two are similar and unless you know the person that may have Narcissism it can easily be mistaken for high self esteem.
Narcissism is when a person has an excessive need for admiration, disregard for others' feelings, an inability to handle any criticism, and a sense of entitlement.
High Self Esteem is when you believe in yourself and know that you are a great addition to this world.
When you have high self esteem you also recognize that you might have a few weaknesses, but you also have a lot of strengths which really shape up who you are.
High Self Esteem ultimately comes from living your life in a way that aligns with your values.
Narcissism and High Self Esteem are totally different but unless you know the person it can be mistaken for each other.