On October 23rd nearly 84,000 Americans tested positive, a record. 
—— a record 应该是前文全句的同位语，相当于 which was a record. 同位语可以用主表相等、交换位置去检验。
Only about 40% of voters plan to cast ballots in person on election day, though, a record low.
——网友理解正确。 low 是名词“低值”。a record low.：历史最低值；记录最低值。
For 2021 the number is 40% higher: a record 235m. 
——网友理解正确。加 of 更明确是同位关系。