This is expected behavior when the value of the filtered field is
null. From Notes about Filtering on Types of Fields and Values:
Filtering on Blank or Null Values
When you use the “less than,” "greater than,” “less or equal,” or “greater or equal” operators on fields that have numeric values, records with blank or “null” values are returned as if their value is zero (0). For example, if you create a workflow rule or a lead assignment rule for accounts with the criteria Annual Revenue less than 100000, account records match if their Annual Revenue is blank.
To limit results to records that are blank or contain “null” values for a particular field, choose the field and the “equals” or “not equal to” operators, leaving the third field blank. For example, Amount equals returns records with blank amount fields. You can search for blank values or other specified values at the same time. For example, Amount equals 1,,2 returns records where the Amount is blank or contains the value “1” or “2”.
null field is treated as if it had a zero value, and therefore matches the filter. You'll need to add additional criteria to exclude records with
null field values.