Well, Spock kind of painted himself into a corner when he and McCoy told Kirk that the only problems during his absence were minor ones that were "inevitable when humans are involved". Then Kirk asked if his last orders helped, and Spock would have had to admit that he and McCoy were having serious conflicts until the taped orders helped them resolve them. Basically, it was a case of Spock's human half peeking out just a bit and leading Spock to tell a little white lie to avoid some embarassment. Not a very Vulcan reaction, perhaps, but certainly a human one.
Spock's human half definitely helped him to tell the occasional lie when the situation demaded. I can't imagine a full Vulcan, regardless of the stakes, telling Norman that logic is a little bird, chirping in a meadow, or that logic was a wreath of pretty flowers which smell bad.