Approximately 5% increase and is inline with inflation.
https://www.law.cornell.edu/uscode/text/26/402
Everything, including the rounding to a multiple of $500, is specified in the law.
[1] https://www.aei.org/wp-content/uploads/2020/01/cpi2020-875x1...
For a balanced take: https://www.investopedia.com/articles/07/consumerpriceindex....