This is because the total of the 5k samples were actually not that new in relative terms and used many samples from the KN3000 era albeit with double the wave rom. The superior sound of the 5k was mainly due to the vastly improved speaker/amp system with the built in sub-woofer.

The 6k used many newer and more sophisticated samples - again in relative terms, one aspect of this advance seen in the newer soloist sounds. This was built on and expanded in the 6k5 and 7k which thus represented a step change in overall voice quality compared to the earlier generations.