Question related to articles when using with main, major, and key [closed]

When I see the phrase 'The main advantage of something', I know the speakers imply that there is only one advantage. However, in the two following phrases 'a key advantage of something', and 'a major advantage of something, why is the article 'a' used instead of 'the' in this case? I think 'main', 'key', and 'major' somehow indicate the uniqueness of something. Thanks for your help!

The speaker uses 'the' to indicate that there is only one key thing. In my opinion, the speaker indicates that there could be more than one key thing by using the indefinite article 'a',and he is talking about one of those key advantages.