Why did the weapon change?

I was so confused because they were making it a big deal about the main murder weapon being a knife, even having the witness draw it and comparing it to a previous drawing. So why towards the end of the show the weapon was an ax? I think at some point they showed the younger brother with the knife and the ax but then the knife was never brought up again.

I have the same question, the victims or their family also drew arrow knife and they showed seonho with it 2-3 times but then the weapon was an axe?