The browser demo page says "Dextro supports up to 4 objects to be detected concurrently when used via the full API.", I think that's the problem.
I want to clarify: the 4 object concurrent detection refers to 4 classes of objects. On the Experiment page, you can only choose one class to detect on (whether that is person, bottles, cars, etc). However, by using the API, you can simultaneously search for cars, planes, people, and motorcycles, for example.