Actually, this phenomenom has been discussed at some lenght at cdfreaks. It is a firmware bug. It should only occur with certain mid's. It is a generally accepted belief that the 1620 should be able to read all dvd +/- at 16x, but for some unknown reason it has been limited in the firmware........
"Why? Said the spry little fly."
Nec 3500 LD V2 B6 Quiet
Benq 1620 B7T9 Stock
Liteon 1633sx BS41 Patched for 12x DL Ripping