$db.DiskSizeRequired vs. $site.Usage.Storage ?

I have a large content database that contains 1 site collection. PowerShell shows me the following. Why the discrepancy? I suppose this is another way of asking if these properties are documented anywhere? I haven't been able to find anything.

$db = Get-SPConentDatabase -Identity WSS_Content_My_Database

$db.DiskSizeRequired/1GB
214.9296875

$db.Sites.Count
1

$db.Sites[0].Usage.Storage/1GB
396.401170487516

Thanks.