It is my understanding, at least for the Tour de France, that the category a climb is classified as also depends on where the climb occurs during the route that day. For example, a normally categorized "2" climb that occurs early in the day could be up-categorized to a "1" climb if it occurs near the end of the stage. That takes into account the fact that people are "worn out" and "tired" later in a stage when the same climb would be more difficult than if it were attacked when everybody is fresh.

On a side note, I've noticed that Strava has downgraded several local climbs in my area. Several that used to be Cat 3 climbs are now Cat 4 climbs. Anybody else see that in your area?