Most likely it has to do with the invoice setup.
Example: Invoice Templates for this Facility (and several other facilities) may have criteria Auto/Manual for time sheets. This setting means include TS imported Automatically and then Manually edited - this is what A/M is. You may not have imported TSs (you may have manual checked for the invoices, but setup in Admin had both checked and it may not match). I think it's better and safer to ALWAYS have all 3 options checked in the invoice setup: Manual, Auto, Auto/Manual - this will cover all situations. If different items are checked on Invoice filter.