Overview:
The player cache is a dictionary that saves the player statistics from a previous scan result.
Benefit: Speed. It's faster to look up a players statistics in such a dictionary than to bug the database again.
Drawback: You won't get the latest player statistics if you are importing new hands at the same time...
You can clear the player cache manually and force Table Scanner to get the stats from your Database.
Also, in "settings->auto clear player cache" you can enable the Table Scanner to auto-clear the cache before every scan...