The knife in fact doesn’t match the wound.
All that proves is that at one time she used the knife.
I am surprised we haven’t heard more about the blood mixing though, for that is decent evidence.
Experts testified at trial that the knife found at Sollecito’s apartment was compatible with the wound that killed Meredith Kercher. A second, smaller knife was used as well. This is the knife that left the imprint at the scene.