I doubt VW was intentionally "hiding" anything in the premininary evaluation (PE). They are likely simply reporting what was requested by NHTSA. The initial data request was a PE and the data requested was very specific to vehicles "stalling" likely because that's what consumers documented in their report of their incident to NHTSA.
For example, in the PE the defintion of "alleged defect" was "stalling and/or loss of motive power". So if the car simply wouldn't start and was fixed under warranty by replacing the fuel system, the failure would likely not be shown.
Contrast the above definition of "alleged defect" to NHTSA's latest definition as shown below. I believe we will see more HPFP failures being reported now than was shown in the PE. I think it's safe to say the original numbers of HPFP failures are low. Notice the word "stalling" is not now mentioned. There have been cases where vehicles simply acted up briefly while driving (no stalling), simply didn't start (no other warning) and so forth that resulted in HPFP replacement (but with no stalling).