Imagine an antivirus program that looks at a piece of code and outputs either “Yes, this is malware” or “No, this is not malware.” It is not perfect, but it is pretty good.
If the malware authors have access to this program, they can test their malware with it. They can keep modifying their malware until it passes the antivirus program.
Once the antivirus people publish a function AV(code)→boolean, the malware people can use that function to make malware that the function mistakes for non-malware.
If you publish the exact metric that you promise to use to make a decision, then people who want to control your decision can use that metric to test their methods of manipulating you.