It's probably because the funds aren't priced at the beginning of the day, but at the end. With January 1 being a holiday, the 1/1/2020 listed balance is actually the balance at market close on 1/2/2020, which would only match the 12/31/2019 balance if the underlying instruments didn't change in value at all from 12/31 to 1/2 - very unlikely.