getting Content db size using powershell is less than the actual size

Hi ,

I am using this simple script to get the actual size of each content database

$ContentDbs=get-spcontentdatabase -webapplication <web-app url/name>

foreach($contentdb in $ContentDbs)
{
$ContentDatabaseSize = [Math]::Round(($contentdb.disksizerequired/1GB),2)

}

But the size I am getting is at least 10 to 15 gb less than the actual size of the db in SQL server.

Why is it so. I confirmed that the total mdf file size in database server is at least 10gb more than the powershell output size.

Please let me know if any one has any information like that or please verify at your end