Hi,

The basic calculation is based on state, and model category. If a parent is defined, the parent's model category is used. 

For the records showing as parent = empty, is the field truly empty? If you're not an admin, you may need to ask them to inspect the XML to see if a broken sysid value is there which will break the reporting ability. They should fix the data using a fix script to remove broken reference values.

If you have access to Now Support, the following article documents how SU's are calculated

https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB1122350