"When there is a large discrepancy between the HPI benchmark price and the median or average, it typically means that there's been a change in the composition of sales."
Westside median detached prices can oscillate as much as $400K from one month to another -- this doesn't mean the price level has changed by this amount.
In other words, either proportionally more or less high priced homes being sold. This is one of the reasons the MLS Home Price Index was created--so that unsophisticated Realtors, media and consumers aren't misled by choppy price data.