Because it's not like marbles. It's not a math thing.
It's more like a canary in the coal mine. If the canary dies, you should check on the air quality. If they went through the trouble of removing the brown m&ms you have a fairly strong indicator they actually read the document in the first place. And a strong indicator they have a good attention to detail.
Exactly, a concept also known as a "litmus test", if you pass the test you may still fail the actual, but if you fail the litmus test there's more or less no way you'll pass.
It's more like a canary in the coal mine. If the canary dies, you should check on the air quality. If they went through the trouble of removing the brown m&ms you have a fairly strong indicator they actually read the document in the first place. And a strong indicator they have a good attention to detail.