Home value on zillow?

For the past three years I have tracked my home's Zestimate on zillow fairly closly.The historical data for my area seems skewed as of early June timeframe. The Zestimate for my house in June 1st, 2009 was $348,000. Now the chart shows $322,000 for the same date. I noticed this blip in early June when my house droped from a Zestimate of $327,000 on 27 May 2011 to around $283,000 in early June 2011. I have the email updates from zillow to prove it. Can anyone explain this?