Concepts
Half-life vs detection time
Detection windows depend on test sensitivity, metabolites, chronic use and more. Half-life alone cannot tell you if you will pass a test.
Different questions
Half-life describes how quickly levels fall under a model. Detection windows describe how long a lab test can see a substance or its metabolites above a cutoff. They are related but not the same thing.
Why windows vary
Factors include dose, frequency of use, body composition, liver and kidney function, the type of specimen (blood, urine, hair, saliva) and the specific assay used.