Automated appraisals have to use some values to calculate value of a domain. Most reliable numerical values are search volume and CPC and these are not static values (if I don't consider TLD, age and other major static values. Traffic is not a reliable factor, shouldn't be considered unless it's type-in traffic.).
Monetary value of domains -like every commodity- are not really static. All prices have changed since 2015, not only domain prices.
Unstable CPC rates and search volumes are just a factor.
Other major factor in unstable domain prices is new trends in domain market such as crypto. Each market has a limited size. Total market size in each industry moves slowly as it is affected by countless variables in macroeconomics. New trends actually take their most of their share from the same pool (market size) and this causes lower prices in non-trend (non-crypto in our example), old fashioned domains.
To explain better I will give an example from computer industry: Laptops lowered desktop computer prices, smart phones lowered laptop and desktop prices. Because computer market size has a limit and the market doesn't grow as fast as the speed of invention in newer computer versions. If you buy a laptop, you don't buy a desktop (or you buy 1 less desktop), if you buy a smart phone, you buy neither a laptop nor a desktop. If there were no laptops and smart phones, desktop computer prices would be much much higher today. If you buy a crypto domain, you don't buy a one word or 4L domain (or you buy 1 less domain). Because you have a limited budget. Budget of all consumers = total market size.
One of the scientific explanations that could be considered to understand the basics:
https://en.wikipedia.org/wiki/Substitute_good