The DVD specifications say that a dvd should not produce (the errors are not so much on the disk as they are created by the readback process) theses error rates when read:
1) No set of 8 consecutive ecc blocks should have a combined total PI error rate of more then 280.
2) Each ecc block should not have more then 4 PO errors. Note that most drives only report PIF, which can be ussually be used as a good appoximation of PO.
Now, the difference is that the Liteon drive reports statistics for each ECC block read, and the software tallies it up, displaying combined 8xECC block readings for PI, and single block statistics for PIF errors. This means that each PIF reading needs to be 4 PIF or less for a passing DVD.
For the Benq drive though, the drive only returns statistics for each set of 8 ECC blocks. Therefore the PIF reading can be up to 32 and the DVD could still pass the DVD specifications. The author decided to make 16 the cutoff though, because as PIF increases beyond 16, the likelyhood of a single block being higher then 4 becomes greater.