Back when we were still compiling pandemic data at the Covid Tracking Project, we documented all the federal sources for stats, and we compared the two fatality-tracking data streams in detail here:

https://covidtracking.com/analysis-updates/federal-covid-data-101-working-with-death-numbers

…most of the difference was about timing—the provisional death data stream is slow because of the way it's assembled, and faster data is really nice to have. But the NVSS system is still reporting—and better standardized than the compiled data from states was.