A knowledge base article about How does Duo know how to choose what device to use to complete a 2-Step? provided by the UC Berkeley IT Service Hub - Knowledge Portal
After entering in CalNet credentials, the browser will send an authentication prompt to a registered device using the most recent method as the default. If you are using an incognito or private browsing window with no history of 2-Steps, Duo will choose the registered device it considers the most secure as default.