Black bow attunement error?

A have a friend that build a level 8 Ranger with the hunter elf that has a +16 to attack. He his se the black bow and a 20 in dex. 

The only thing that doesn't add up is when he attunes the black bow it adds +6. 

I do not know why it adds +6 it should only be a +1 to attack 

https://www.dndbeyond.com/characters/6461758

Can anyone explain what I am missing?

Might want to ask the homebrew's creator. It looks like it gives a +8 to hit, but no +1 to damage. So, yeah, very not as it says.

Thank you. I did'nt realize it was a home brew item.