In this statement, how is the location of the parties signified?

"In the gallery above the sales department, supervisors can monitor the customers without disturbing them."

I’m taking a test which has asked me to state the location of the customers. From my interpretation of the statement, the supervisors are situated in the gallery while the customers are in the sales department; the correct answer is that the customers are in the gallery and vice versa.
Can someone explain the reasoning behind this?