Jumping from 51cm to 60cm, to me, suggests they sold the sizes in between and have yet to restock them on the sales floor ..

I don't think that's the case. At that site you build your own bike within the constraints imposed by their choices. They only show two sizes and only for one frame that can be used with that Pinion. Maybe they will offer more in the future.

I've spent time at that site last year and if a particular frame was not available, you could still 'create' the bike you wanted though you would have to wait for the specific frame to arrive. I wanted a brushed aluminum frame for a specific bike I 'created' online, but they couldn't supply it. After months of delays I order the bike I'm presently using.