This could indicate that the TRT is not performed with the "accuracy" setting.... - if you choose any another setting, it will drop data during reading, so there's room for error (false negatives)! A good TRT indicates a fully readable disc (in the drive used to perfom the test) only if it's set to "accuracy".
Another possible cause is that you perform the TRT in one drive, and try to read the disc in another drive. Many marginal discs/burns can be fully readable in drive X and show unreadable zones in drive Y (which is the reason wy we are several on this board to choose our pickiest reading drives for performing TRTs).
If you did all your tests in your Benq, well, yes I'd say that it's a defective drive. - what's your PSU (power supply)? Some low quality PSUs can damage optical drives quite rapidly. :a