They usually do this to fix the "fireworks" time. For example, if park closes at 8, they will list HEA at 8, reserve dessert parties for 8pm. If then they change to 10 pm closing (very likely), the fireworks will remain at 8 pm. If they change to midnight though, then fireworks will probably be moved up to 10 pm (not so likely).
They usually change 8 pm closing to 10 pm closing, and maybe add evening 10-12 am EMH. And if they have 10 pm closing they probaly update to 12 am closing. Sometimes, in special very crowded weeks, they can add up to to 4 hours of extra park time, including both extending general hours and adding EMH.