[tominous]

Third option: the test ID is generated well before the test is taken.

When I donate blood the nurse pulls out a sheet of around 20 pre-generated identical barcode stickers, and attaches one to each piece of paper and bag of blood. This uniquely identifies my donation end-to-end.

Djokovic's PCR tests were done at two separate labs. It is quite possible that each lab is bulk-allocated a unique range of IDs every day or week from the central authority. You'd see big "time jumps" from one lab to the other if you assumed the ID was always in time order.

Depending on how they handle their range (e.g. if they are handing physical stacks of barcode stickers around) the code may not even always be in time order in the same lab.