You are comparing apples and oranges. First of all, the scan done with the Pioneer DVD-ROM is just a simple Transfer Rate test. The test run on the BenQ is the Disc Quality Test, which operates in a different mode. If you run the Transfer Rate test on your BenQ, you will get a similar graph without the descending lines.
As to why the descending lines on the Disc Quality Test, some BenQ drives seem to do this and some don’t. This was reported in the NEC forum testing beta firmware and using BenQ 1620s for testing PI errors. Dee-27, the moderation and thread author has two BenQ 1620s, one of which does the lines and the other doesn’t. Both drives perform normally in her opinion. That was her answer to another BenQ owner who reported the lines to her and asked if he had a problem.
My drive usually does one downward line near the beginning of the Disc Quality Test, but in testing a commercial DL disc to check the results I had three of these downware glitches. I don’t know the reason for it, but I would disregard it. If your drive is O.K. otherwise, I would consider this behavior normal.
If you want straight lines, run the Transfer Rate test.