The quality (or readability) of a disc is greatle dependent on the drive that does the reading.
Because the digital signal is modulated as analog variatons on the physical cd-r medium, the quality of the reading can vary quite a lot too.
Some drives are better at reading certain types of burns (i.e. certain media burned on a certain drive), while other drives excel on others.
For your Toshiba drive, the Arita discs are probably still in ok shape. For your Plextor they are not in equally good shape.
So, one should keep in mind that the disc read quality is drive dependent, although certain type of errors will become read errors regardless of what drive is used for reading (big enough scratches, holes, etc).
Did that help at all or confuse you more?