Fez, thanks for being one of our athletes. This issue is addressed in our document Understanding Your 8020 Run Plan but I have included the relevant section below for your convenience.
We actually want you in Zone 3 and higher for 20% of the time, and work to eliminate or reduce Zone X.
Why Zones X and Y?
You may wonder why a seven-zone intensity scale such as ours tops out at Zone 5. The reason is that in the original version of the scale, Zone X and Zone Y were not explicitly named. Instead these zones existed only as gaps between Zones 2 and 3 and between Zones 3 and 4, respectively. The first gap was created to ensure that low-intensity exercise efforts did not accidentally bleed into moderate intensity and the second to encourage athletes to commit to either moderate or high intensity. Nevertheless, many athletes found the gaps confusing, so we modified the 80/20 intensity scale in a manner that eliminates gaps and the confusion they cause while preserving the distinction between untargeted zones (X and Y) and targeted zones (1-5).