It increases retrieval cues. The more you recreate the study environment, the easier it will be to retrieve the information. It's also the reason it is recommended to study in the room you will test in, if possible.
Its true to an extent. Also another thing to keep in mind is if your one of those students who gets well dressed up for class but shows up for an exam in sweats and a baggy shirt, you could potentially decrease your test results.
Does that have the same effect as the gum?