It's because of the colder weather.
I just had this happen to me over the weekend. I was pulling in third gear trying to get a max boost reading and I got the exact same thing. I didn't take into account that the temperature dropped overnight from 90degrees Friday to around 60ish on Saturday. Turned the boost down a bit, and it hasn't occurred again. I'll probably have to re-tune it again when the temperature drops even more.