Correct, VM_DELTA performs a datastore browse to look for .vmsn and -delta.vmdk files as these can be left behind and not properly show up in the snapshot manager. So if this flag is enabled and you have files that have creation dates that are either (15,30,60 days old) which are the defaults, they should show up.
Per the documentation:
There are 54 parameters that can be configured to display various categories of the health check report. By default, everything is set to "yes" except the following 4:
CLUSTER_PERFORMANCE
HOST_PERFORMANCE
VM_PERFORMANCE
VM_DELTA
as these can potentially increase the amount of time the report takes to generate.