As someone who has 3 elite characters (all trappers) I can say that having a mastery does not mean you are some godly player who never loses with that character.
If someone has elite character it means he really likes playing with that character and he, in fact, did spend a lot of time playing with that character. Maybe he worked towards those masteries, maybe they just came naturally while he was playing. Doesn’t matter, he played him a lot.
People with elite characters know how to use that character, they know how to use all of his abilities, not just spam single ability/weapon whole game. They understand their character’s strenght and weaknesses. They do however still make mistakes. Like the rest of us, they are still learning the game, trying to get better. You should not expect someone with elite character to be the best player on the server, but you could expect him not to make usual beginers mistakes and are generally quite fun to play with/against. Also, having 10% buff in all abilities is really, really nice.