If you change your story, the probability of your guilt is 1.00. If one version of your story includes a false accusation (i.e., of another person), that goes up to 2.00

At most you could convict them of something like obstruction of justice or criminal defamation, if you could actually prove mens rea beyond reasonable doubt.
But convicting somebody of murder just because they made an incorrect or even deliberately false claim, would not serve the interests of justice.

In this case, Knox was actually convicted of "calunnia" against Lumumba.
It could be argued whether this conviction was fair, since Knox made the claim in a statement signed at the end of a lengthy interrogation without an attorney, in a language she was not familiar with, and the statement mentions that she "remembered confusedly".
Anyway, fair or not, she already served her term for that false accusation.