While you are correct that there is a pattern of steppings, this is entirely coincidental and reflects additional steppings that were required for POWER8E, and then even more that were needed for POWER9.

Preproduction samples of POWER9 are generally DD1.2 (now unsupported by firmware), DD2.01 (second prototype pass, supported but with serious defects), and DD2.1 (almost fully functional but with defects requiring another stepping). DD2.1 is what we're basing our no performance loss comparison against.