Feathercoin's difficulty algo is seriously broken. Lots of high freq swings ranging from 10 to 50(!). You should have been asking some folks who know about Feedback Control Systems...
The difficulty changes to compensate for the changing hash rate as different multipools switch coins. Unfortunately, you can't see into the future, so it is a balancing act, looking at the latest and long term block rate. Other devs at the time of the introduction / fork, retained the "difficulty change damping", which I was against, which would have made eHRC react faster.