To answer this question, we must clarify what really happened at the end of the novel rather than what the adults pretend has happened.
In reality, Bob Ewell snuck up on Jem and Scout with a switchblade—a weapon he did not have the chance to actually use because of the intervention of Boo Radley. The kitchen knife found on the scene belongs to Boo Radley; it was this knife that Boo used to stab and kill Ewell as a way to protect the children from being harmed.
However, in order to protect Boo and to ensure that he is not charged with murder, Heck Tate hides the switchblade that Ewell possessed and makes it look like the kitchen knife was what Ewell was planning to attack the children with; this makes it seem like Ewell fell on his own knife rather than being attacked by a third party.
In this chapter of To Kill a Mockingbird, Scout is in costume: she's a ham, so she can't see or use her hands. So, Scout feels around with her feet. She pushes around on Bob's body with her feet: she feels a belt buckle, buttons, and "something I could not identify" (the switchblade), and then a scratchy face.
Heck finds Bob stabbed with a kitchen knife. This is the knife Boo Radley used to kill Bob Ewell.
Later, Heck Tate shows Atticus a switchblade he claims to have taken from a drunk man that night. In reality, this is the switchblade Bob Ewell tried to attack the children with. Heck Tate pretends he got it from a drunk man earlier; he intends to make it look like Bob Ewell tried to attack the children with the kitchen knife and fell on it. The switchblade is evidence that the story he wants to tell—the story that saves Boo from daunting public scrutiny—isn't true, so he fabricates a new origin for it.
The kitchen knife could have been Arthur's ("Boo"), and Heck took the switchblade out of Ewell's hands when he found him dead, so that there would only be one weapon at the crime scene, and no evidence that another person had a knife.