Had some time to look over this claim. You read the chart wrong because Genesis prices increased 4.19% over the past 90 days. Here's the chart:
Used Car Price Trends - CarGurus
Where the price did go down was YoY (Year over year- compared to 2019), which makes sense because 2020 is 3 years from 2017, which is when Genesis officially launched its own branded models. 3 years is the typical time of a lease. That means that there's now a flood of Genesis-branded products that weren't there before, because the ones that were in 2019 and prior were likely early lease returns. If you really want a true view of price differences YoY, you would have to add the 15-16 Hyundai Genesis in there, which isn't counted by Cargurus as a Genesis.
Also, because Genesis prices are counted from 2017 onward, you would expect them to go down each year because there would be more older and cheaper cars being counted each year. Other brands have been around long enough where YoY makes little difference.
As I side note, I work with a lot of numbers in my job, and you can't "add percentages". According to the chart, the percentage is relative to each brand.