It's also strange that Deneb IV ("Cygnus IV," in earlier script-drafts) would represent the ultimate, unknown edge of UFP-explored space, since the Deneb system was mentioned practically every other episode in TOS. Maybe back in Kirk's era, Starfleet avoided sending any expeditions beyond it, and simply worked in the other direction instead?
Although it works out nicely, because we now know that Deneb is much, much farther away than we used to think it was; of all the real named stars mentioned in Trek, it's probably the most distant (even more so than it was known to be when "Farpoint" was written).
Luckily "Deneb" is a word found in several stars' names. Star Trek Star Charts
assumes that the Deneb mentioned in TOS is actually Deneb Kaitos (aka Diphda or Beta Ceti), a much nearer star.
Anyway, it's not all that surprising that they used the star name in contradictory ways. TOS had done that before. How many conflicting references to Rigel were there over the course of the series, not to mention subsequent series? Roddenberry liked using familiar star names whether it made sense or not.