I know it's dicey regarding Spock lying. Vulcans don't do it often, but they can.
He's omitted the truth, exaggerated the truth, but flat out lied like here and not for any real reason?
In several other eps. he lied - The Enterprise Incident
, The Menagerie
& movies: Star Trek II: The Wrath of Khan, Star Trek VI: The Undiscovered Country
but those could be seen (I guess) as for the good of the ship/crew. Why the lie in this ep?