You are kind on calling it "unusually strict"... it is just an illogical system and a waste of papers.
The sad fact is that once you arrived at HKG, one of the many stops was the document check. And yes, they spent times going through all your papers because you have to wait for your test result anyway. No one is in a hurry.

Sadly HK government did not really expand on the "document check" part. It might be a mistake with the type of PCR tests taken or wrong dates booked for quarantine hotel, or something simple as not providing the right certificate for the PCR tests taken in SIngapore. They are obsessed with the type of PCR test that you take and you must provide that certificate that proves the test is certified.

Carfield