I try running three different searches:
1) project = MYPROJ AND component in (" COMP1 ", "COMP2") AND status = Open
2) project = MYPROJ AND component in (" COMP1 ", "COMP2") AND status = Open AND "Requirement Id" is not EMPTY
3) project = MYPROJ AND component in (" COMP1 ", "COMP2") AND status = Open AND "Requirement Id" is EMPTY
The results:
1) 95 results
2) 29 results
3) 14 results
I would have expected the sum of 2) and 3) to be 95, but it isn't. It looks like the results of 3) are not complete.