...Because it didn't matter? The point was that there was only one death penalty, and its diary number was irrelevant.
I'd much rather it really were GO4, though, because
a) it makes no sense to quarantine Talos IV after the events of "The Menagerie"
b) as noted, death penalties do come and go, even in the latter half of the 23rd century
c) Chekov isn't known for verifiably erring, the Russian jokes notwithstanding - in fact, Spock is the one known to make erroneous statements every now and then, while Chekov is chided by him for inaccurate or incomplete (rather than erroneous) facts so often that he probably is extraordinarily cautious now!
Timo Saloniemi