Source taken from the Nero CD-DVD Speed manual:
To make perfect copies a drive should be able to read data from the leadout (see offset explanation).
All drives start reading audio sectors at a different position from the actual start position. This offset is usually a few 100 samples. Also recorders don't start writing at the start position. Nero CD Speed reports the total of the read offset of the tested drive and the write offset of the recorder which was used to create the test disc.