You want to know why you are seeing customers that have been marked as TAXABLE showing on a search for customers without certificates.
This is expected behavior for the search.
However, you can exclude them from any search by using the Advanced Search options, and under Customer Exempt/Exposure reasons, select "Does not have" and "TAXABLE":
You can also search specifically for any exposure zones you are looking for, such as Missing Certificate.
Please note that for any campaigns, the customers marked TAXABLE will be excluded automatically.