Star notes, or replacement notes, are often printed in advance of regular notes. The BEP does this to have them on hand in case they’re needed to replace printing errors. Different “runs” are required to accomplish this. Some errors are discovered that require an entire sheet to be replaced and some errors require packs of replacement notes to be used. Large runs of pack notes are typically easier to find than replacement sheet notes.
This link takes you to a website that documents BEP monthly totals for newer, modern notes. It chronicles the different denominations with the month printed, the amount printed and whether they were printed in Washington DC or Fort Worth.