To clarify, Theme Scientist operates on a time-based approach rather than splitting live traffic. It is advisable to set the time increment for theme or product tests to a few hours, such as 3-4 hours or more, instead of switching every 30 minutes.
When a visitor is on your store, and Theme A is being displayed, if the schedule is set to change to Theme B, the visitor will continue seeing Theme A unless they refresh their page. Only upon refreshing the page will they view Theme B. This behavior is due to Theme B being live in the database.
Additionally, if a user adds an item to their cart during this time, the event will be attributed to Theme B because it is currently live in the database. Therefore, we recommend using larger time increments to avoid such discrepancies, instead of switching themes every 30 minutes.