Because averages are only 30 years, and storms do not distribute equally. RDU probably takes the most accurate measurements as well. There are two airports in Cobb county that send in data to FCC, but somehow there is no reliable average snowfall for either one. If you believe what is on paper, KATL averages more snow then here in Cobb, but that just isn't the case.
This article shows the 7 biggest storms Cobb County has had, and several of them are since 1991, which would be included in a 30 year average if there was one.
AJC
March 1993 - 15"
December 2017 - 10"
January 2011 - 7"
January 1991 - 5.5"
KATL's largest snowfall ever was like around 10 inches in 1940 or so, and they only received 4" during the 93 Superstorm, as well as 2" in December of 2017.
If you just used these 4 storms and nothing more, the current 30 year average would be
1.25", but there have been many more storms, and the data is just properly collected.
Since I moved back in 2017 these have been my totals:
2017-2018: 12"
2018-2019: T
2019-2020: 2.5"
2020-2021: .5"
2021-2022: 2"
My current 5 year average is
3.4", which is probably what the yearly average would be currently if better data was collected.