Students are eligible to showcases based on the requirements explained here.
Students may have added a showcase manually before meeting the requirements. This can either show up on in their Project Library as a Blank Canvas project or another project that was manually added from the showcase-all page. If the student deletes the extra project that was manually added they will get the prompt to once again choose a showcase project to play with.
This happens because the Hatch platform enforces a certain amount projects based on the requirements they've met on their certification page. You can learn more about those requirements here.