Hello Herculean, the current market is hosting a huge investor community, especially in HERCLES. In some cases the investor can buy and "FLIP" the property after the home has been vacated, cleaned up and perhaps updated.
There is a home at 1583 Partridge, Hercules, CA 94547 (Birds Community) that sold for $35,000.00 this year. The home was totally trashed, needed roof repairs and red tagged by the city. An investment group purchased it, evicted the current tenants, cleaned it up and sold it for $365,000 (it sold in just 39 days!) within months of the original purchase, just know that a home that has undergone this process will need to be owned by the investor/seller for a minimum of 6 months, if the buyer is utilizing FHA financing, for conventional financing, there is no wait time to resell.
My personal research of the current state of the Hercules Market indicates that prices are moving in an upward direction. I personally listed and sold a single family home in the Bayside Community for $350,000 in March of 2009, today that home would go for about $400,000.00; this is due to the vast inventory of REO homes that were on the market in the last quarter of 2008 and the first quarter of 2009. What you are experiencing are the effects of “Supply & Demand”
Best of Luck in your home search!

There could be several reasons behind this occurrence. In Florida, I have seen several instances, and probably the most common in this economy, where a home was REO owned and sold (most likely to an investor), then days or a week or two later, relisted and on the market for a considerably higher price.