As someone who works in this area, I have some points:

1. More often than not, things like this are dictated by need and budget more than vanity. Sure, if it looks like a cluster might be close, a customer might press a vendor for a few more compute nodes to push them over the edge, but even that’s fairly rare. For the customer, being in the list at all is decent marketing fodder. For the vendor, having the top spot is cool, but having more systems on the list than your competitors could be argued as at least equally as cool.
2. This list covers systems that are publicly known about. It’s long been known that institutions like the US NSA have some impressive computing power whose specs are classified. They aren’t alone. Plenty of similar installations around the world, and not just in government. Financial companies, for example, can be quite secretive about their resources.