One of the study's authors told the Guardian that the hardest-hit areas were non-metropolitan counties, especially in the west and the south, with fewer resources for investigating deaths (and lower testing levels) — as well as different methodologies for assembling the official numbers.
There were also people who just refused to acknowledge loved ones had died of covid, and actively tried to block death certificates from listing covid. This is discussed in for example this article . In many cases, the official records simply used whatever family members said was the cause. Some were even more extreme. From that article:
In Cape Girardeau County, Missouri, coroner Wavis Jordan said his office “doesn’t do COVID deaths.” Jordan does not investigate deaths himself. He requires families to provide proof of a positive coronavirus test before including it on a death certificate. In 2021, he hasn’t pronounced a single person dead from COVID-19 in the 80,000-person county.
While part of this is due to lack of resources, and both TFA and the above linked piece discuss some of that, part of this is political in nature, with one end of the US right-wing deciding that covid wasn't a major issue and thus downplaying covid deaths. Unfortunately, downplaying a disease for political reasons doesn't make it less deadly.
On the eighth day, God created FORTRAN.