Reading the rest of your comments today, I'd doubt you'd mix up the warnings! Had to ask just to confirm 'cause that confusion when something happens suddenly has been seen before...
Mike's answer sounds like a real possibility. I'd guess that even if it samples continuously, there would be a "lag" built in to avoid making changes too quickly trying to respond to transient variations in air pressure (averages over time) & that steep grade fooled it.
At least you don't need to have the car re-tuned for high & low altitudes, as was the case before electronic systems!
"Rubber Side Down"
S.R. #275 Focus Takeover #003