The paper touches on that:

> The ARR shows the extent by which total predicted COVID-19 deaths exceeded officially reported COVID-19 deaths during the period. A limited number of counties had ARRs < 1, which suggests that there were more officially reported COVID-19 deaths than total predicted COVID-19 deaths. One reason that a county could have an ARR < 1 is if death certifiers recorded people as dying from COVID-19 when they had COVID-19 but actually died from another unrelated cause.