I read that the judge said jurors would be confused... is this normal in any other type of case? Not a satisfying explanation.
So on appeal, the appellate court can overturn a specific finding (i.e. one of the underlying claims are invalid) but not overrule the overall finding (i.e. the infringement finding is still valid because they found that if certain other claims were valid then the defendant did infringe).
This is obviously more complicated in practice because juries are supposed to only decide issues of fact and not issues of law - but in some cases it's difficult to say exactly what is law and what is fact. This is especially so in patent law, where the actual letters patent are supposed to be interpreted in light of the relevant patent legislation, and often turns on interpretation of words (which are questions of law).
This is, by the way, why patent trolls like jury trials. It's easy to bog down processes in these trials and confuse the jury.
As for the Samsumg case, that's complicated by a generally ignorant foreman who seems to have convinced fellow jurors that his interpretation of the law was correct.