Bit Kid

I'm trying to understand how I got a very powerful dagger called "Wendigo's Claw" 28 atk from the first boss. I killed him, so a red chest appears and... Wendigo's Claw

Is this rng? Did i do something incredible? I did not understand...

I tried to kill the second boss repeatedly looking for a new item, but nothing ... Anyway, there is something I still do not understand, can anyone explain me?

(i'm using google translate)

SPOILER WARNING

white text below, spoiler tag didn't seem to work.
[spoiler]Every boss gives a special reward for defeating them without getting hit[/spoiler]