why are there so many posts answered correctly, but not marked as a good answer?

why are there so many posts answered correctly, but not marked as a good answer? can you explain to me how this system works please ? I answered a lot of old posts to earn big points, and I never won them. Did I misspoke my answers? are my answers considered false? I do not understand ... but I see that there are a lot of posts resolved in the open only section and not marked as good answer like me. Thank you for your help.

And please tell me how we do a thread ^^'