Pre-production and prototype builds are a whole different ball game from mass production.
I work in the automotive industry (on the supplier / engineering side) and I see this all the time.
Rickey Gadson's bike was a pre-production build, or possibly even a prototype build.
The purpose of a prototype build is to make sure all the parts fit together as designed, sort out any assembly issues, make sure stuff basically works as intended, etc. The parts are frequently made by hand on prototype tooling. The production tooling is not ready and may not even exist yet. The vehicle may not even be completely at "design freeze" yet. If something doesn't fit together properly, etc they can still tweak the design.
Then there is pre-production build. This is a set of vehicles used for validation testing and they are built using the production tooling, but not necessarily using the production processes. The North American manufacturers call this PPAP (pre-production assessment program). The production tooling exists, but does not necessarily "run at rate" (run at the specified cycle time). If there are problems discovered during validation testing, things may need to change. And it happens all the time.
All it takes is for ONE part to require a design change to screw up the production scheduling!
I'm in the midst of another situation right now, in which the next version of a car that you would all recognize the name of but which I will not name (!), got most of the way through tooling build and into pre-production build where they built several vehicles for crash testing, only to discover that the real world did not agree with the computer simulation, and the vehicle failed side impact! OOPS!
The engineering fix for this requires another brace to be designed, built (with new tooling!) and welded in with something like 40 more spot-welds per side ... except that the existing robots cannot do this in the specified cycle time, so they need 2 more robots per side ... except that those 2 new robots won't fit within the existing line layout, the line has to be made longer with another station to accommodate them ... except the building that this line was destined for, with space already allocated for it, has the existing line layout just barely fitting between the two main aisles, which can't be moved without expanding the building!
The only good thing about that situation, is that I didn't design it! I can place a "somebody else's problem" field around the whole situation.
Production delays happen for a multitude of reasons ... it's called "reality".