Actually, the Pilgrim Office affixes TWO sellos to your credencial.
One goes over the handwritten entries, typically on the inside front cover to signify that this credencial is now CLOSED and may not be used again to seek a Compostela. This prevents 'double-dipping."
The second sello goes in place, as the final sello in your chronological progression of sellos.
People who are seeking only sellos, and not a Compostela or other certificate, only get the second stamp. This holds open the possibility of their changing their mind the next day and returning for a Compostela... It happens.
On the other hand, the "solo sello" folks do not have to wait in the longish queues. Once they tell the security fellow that they only desire the sello as evidence that they arrived at the office and final destination, they are immediately taken care of. This reduces the number of folks waiting.
Hope this helps.