[MSAccessProfessionals] Summary Query Issue

I have a summary query which groups on several fields and then sums several other fields.

If I add another column XYZ and "Group By' that column everything looks good.

if I then change XYZ from a "Group By" to a "Where" column, and add criteria ="teststring", everything still looks good. ie it correctly only sums data where XYZ has the value "teststring"

if I keep XYZ as a "Where" column, but change the criteria to <>"teststring" though it now only sums columns of XYZ that are blank. ie it eliminates all data where XYZ has and value not just "teststring"

Any ideas?