Why Are Star Bills Printed

Star notes are replacement notes.
From Wiki;
A replacement banknote is a banknote that is printed to replace a faulty one and is used as a control mechanism for governments or monetary authorities to know the exact number of banknotes being printed. As quality control finds defective notes in the printing process after the serial number has been overprinted, they are taken out with their serial number written down and replaced with another banknote printed specifically for this purpose, so that the amount of banknote being printed stays the same in each production batch. This saves time and money compared to re-printing exactly the same serial number that was used before. It is rare that the replacement banknote has the same serial number as the original faulty one. A replacement note will have its own serial numbering system that separates it from the normal numbering system.