2019 to 2020 changes in initial addresses selected by states

I am conducting some analysis using the 2020 ACS PUMS, and one evaluation is the changes in the number of initial addresses selected by states. I did a simple comparison of the numbers in 2020 to the numbers in 2019. I noticed a significant difference in those rates, from a low of 80% for Vermont to a high of 93% for Alaska. When I compare 2019 to 2018, I see only minor differences in the size of the initial addresses selected by states. I suspect that the 2019-20 changes are associated with the impacts from the COVID-19 pandemic.

Can anyone provide some insight into this variation? Are there procedures and timing that accounts for the differences?

Thank you.