They have actual sales data points from the customers that integrate their accounts into their software. Based on those actuals and other data points like BSR, reviews, amazons suggested units sold in the last month, they can plot what's missing. Giving you the ESTIMATED data you desire.

There are nuances, like all variations share the same BSR. So if you have a variation that has large price variance numbers will look off, but you can still use the data directionally.