Change in weather? On blustery days, I see a big spike in that value.
I pretty much ignore flights and just care about elevation, as it seems like the barometer in the watch (and iPhone) is easily thrown off vs elevation values are in the ballpark.
Example: two different hikes, both about 950' elevation gain, one is 67 flights, the other 8. 2100' hike, 77 flights.
When running in the neighborhood, same route, can swing from 2-15 flights. Some days none. And generally, I'm getting about 220-300' elevation gains in various neighborhood routes and see these weird swings.
ADD: and "flight" is a weird unit anyway re: homes are 8-9' ceilings, commercial about 12' (iirc). Is a flight elevation, or a slope/gradient value? In the examples I provided, a flight for the big hike is 27' elevation gain per flight.