1.) Nothing new or exciting has happened since last time, in which case I feel pretty crappy.
2.) Something cool has happened, but I don't feel particularly boastful nor do I want to get into a long discussion of why that thing is cool.
3.) Something very amazing has happened, but I'm actually not allowed to talk about it.
I do notice when I return the question, I almost always hear the answer this article suggests ("awesome, best month ever, crushing it"). I guess that's why people always gave me funny looks when I used to give an honest assessment.
Now I just always answer, "Good good, we're pretty close to (some lofty feature or pseudo-pivot that is months away)". It's a convenient way to change the subject. Keep in mind this primarily applies to casual meetups with people I see regularly every 1-3 months but don't consider close friends.