I am not knowledgeable about any change, but a five year average would "penalize" anyone who retired early, either by choice or otherwise, or who were pushed down into lower paying jobs in the recent economic debacle.
Let's say someone gets shucked out of their lifetime career in their late fifties, and starts collecting their 401K at 59.5. You can't collect any SS till you are 62, and even then it's reduced until you are 65. You can collect a higher monthly amount if you wait till 70, but of course you will collect it for 5 or 8 years less, depending on whether you would have started at 62 or 65.