Quoted from johnwartjr:

Keep in mind, the PPS page is what the WMS page was... 20 years ago PPS is maintaining/preserving a historic document in that case.
It was a joke from whomever designed the page for Williams back in the 90s, carried over.
Perhaps they were playing a joke. Perhaps it was tongue in cheek humor. People who collect protos often make a big deal out of the fact that they own a prototype. Protos are often worth more, but generally only to the few who collect them. So, if 10 exist, and 3 die hard collectors that want protos of that title have protos and nobody is really looking for one... it may not draw as much of a premium price.
Protos often have cool features that were cost reduced out, but they also often have flaws.
Some say protos are worth less, because they are 'flawed'. Some proto and sample games have things that were fixed after the proto stage that may not be as 'fixable' on a prototype game - things like the ball locks on Jackbot. On sample games, the balls didn't lock as well as on production games, and many people don't like that game because the balls often dribble out of the locks on proto/sample games.
My prototype CFTBL had hand modifications made to the playfield using a file, and sections of the playfield that had no artwork. To some, that may be a detractor.
My sample T2 has no diamondplate, and the art is pretty well worn off the playfield because the ball guides were hand welded and not ground down - so the chipped up balls wore on the playfield like sandpaper, and 30% of the ink is missing. I'd love to find a nice sample/proto playfield to put in it someday, but where am I ever going to find something like that?
A gently used, likely HUO or very limited route use proto like this NGG is really a gem, because you've got a piece that was likely hand assembled by the design team, and hasn't been overrestored. That is, if that's the sort of thing you are into.
If you just want a really nice NGG, this would work, too.