It's hard to think of any way in which that statistic is actually meaningful. I don't think anybody would reasonably argue based on a single event that the average Norwegian is more at risk of death via mass shooting compared to the average American, for a start. When you have only one data point pretty much any statistical comparison seems fairly pointless.
And yet if you took any significant time period that doesn't include the Breivik shooting Norway's results are just fine. I'd even think if you looked at the last 50 or 75 years (assuming we have good enough data) they'd be better than the US's. And certainly if you included Norway along with other countries with similar policies on dealing with violent criminals.
Which I don't even really think necessarily proves anything much on its own, but at least it's a meaningful data point.