Novelty at the time, perhaps? You might be thinking of "non-obvious" but just because two people invented something doesn't make it obvious. More importantly,

Novelty at the time, perhaps? You might be thinking of "non-obvious" but just because two people invented something doesn't make it obvious. More importantly, though, once the patent is granted it doesn't matter, you're in violation. That it shouldn't have been granted in the first place is a separate issue, and one that almost nobody is going to spend the time and money on to sue for a change.