The Nexus line was named after the number of inches, not chronologically. Nexus 4 = 4 inches, Nexus 5 = 5 inches, Nexus 6 = 6 inches, Nexus 7 = 7 inches, Nexus 9 = 9 inches.
Newer versions within the same number of inches were dealt with by smacking some arbitrary letters like "X" and "P" and sometimes the year "(2013)" in the case of Nexus 7.
Pixel 1, Pixel 2, Pixel 3, Pixel 3a, Pixel 4, Pixel 4a, Pixel 5.
You're mixing in Nexus 6P, of the Nexus line, which ended five years ago.