From: United Kingdom
Since a consumer DVD-ROM or Writer is not a calibrated device, it's measuring the difficult it has in reading - thereby the number of errors reported.
For that drive, it's PI (Parity Inner) and PIF (Parity Inner Failure - could not correct at PI level)
1. PI <= 280 over 8 ECC blocks
2. PIF <= 4 over ONE ECC
The Sony/Liteon DVD-ROM measures at 8 ECC, so the PIF limit is taken as either 32 (OK if there are 4 errors in each block, but not if they are grouped), or a compromise of 16 (allowing an average of 2 per block)
Scanning at maximum speed, you will also tend to see an increase just because the faster read speed toward the end makes it harder to read.
If it has trouble playing video (by definition at 1x speed), then I would expect the scan not to improve much if taken at a lower speed - 4x is a good compromise, as it's about the fastest CLV speed, so even at the end, you're assessing the readability of the media and not the high speed performance of the drive.