Science Puzzle

From Gizmodo.com:

On November 14th, skygazers will witness the closest full moon, or “supermoon,” of 2016. But more excitingly, it’ll be the closest full moon since 1948—and we won’t get another one like it until 2034.

So it's been 68 years since an orbital perigee, but the next one will happen just 18 years from now? I can't think of why that might be. Does anyone out there know?