Why were US job numbers which riled Trump revised down by so much?

President Trump fired the US Bureau of Labor Statistics head after job numbers were revised down by more than 250,000, which Trump claimed were rigged against his administration. The revisions were routine, but the larger changes were in response to more complete data. Data collection is based on surveys from households and employers, with revisions common during economic turmoil. Updates are part of the process for accurate reporting, with past revisions seen under different presidential administrations.