The fascinating question, of course, is "31 more than what"?
“To meet the expanding adversary fighter threat, we will procure 31 additional F-35C relative to the Navy POM submission (and 10 more than the PB-16 plan)".
So the answer is "31 more than a plan we haven't seen" which apparently contained a startling 21-jet cut relative to the PB16 (Presidential Budget, the one published at the beginning of the year).
This is fully consistent with two known facts. Navy Air has been developing plans that fund as few as 12 F-35Cs; and we've just seen a plus-up that, most likely, is a matter of the U.S. taxpayer buying up FY16 jets that the export customers don't want (yet).*
So (clickclickclick on the old calculator) this week sees the Pentagon spending $6 billion on extra jets that the users don't want (at least yet) to avert the start of the good ol' death spiral. You're right, a1bill - for some, the news only gets worse!
* That's on the most-probably-correct hypothesis that the plus-up doesn't boost total production, but awaiting final confirmation on that.