apparently, Field 1 is defined in table 2. You have a one to many relationship and your "count field" is defined in table 1. When you define such a calculation in table1 that refers to the value of a field in table2, it only refers to the value of this field from the first related record. Values in other related records are not referenced.
You need a calculation that selectively "counts" different records based on the values of different fields in your set of related records.
ExecuteSQL is one option to use if you are using FileMaker 12.
If not, you'll need 3 more Tutorial: What are Table Occurrences? of table 2 to link to table 1, each related with a different set of match fields in order to match to records of the specified values so that you can add the counts for assessment and treatment and subtract the count for DNA using the count function.