Software development is not all that different in that regard than any other pursuit where repair is involved. Just like a mechanic cannot fix a car if they're unaware of a problem, or if a reported problem cannot be replicated by them, neither can a programmer do the same for a bug.
This is one of the reasons I download and install the release candidates. NVDA is not a major corporation with the resources to hire a dedicated testing department that can flog the software to death. Having been a programmer, and one in an entity that can and did do that, you still will not find all potential bugs. Even with very thorough test "scripts" the general user public will always find a way to do something that no one could ever have predicted they'd do, and with results that are sometimes as unpredictable.
Being a late beta tester on NVDA with the release candidates is a good idea for all users of the software. And those of you who actually use NVDA as your primary method of access can be a lot more thorough in testing just by doing what you always do than anyone creating testing scenarios could hope to be.
I worry a lot. . . I worry that no matter how cynical you become it's never enough to keep up.
~ Trudy, in Jane Wagner's "Search for Signs of Intelligent Life in the Universe"