Real Estate - Why are some homes listed as sold at a fraction of the market rate?

I’m browsing on zillow.com for homes, and I came across something really interesting. A handful of homes were recently sold for almost nothing compared to the market rate. When viewing the sale history, it shows a purchase a few years earlier for roughly the market rate, but in the last year, all sold for 1/10 to 1/4 of what the market rate should be for these homes.

What is going on here? Are the owners pulling through some loophole in the system? Some tax thing? Is there a tactic to sell a property cheap to say a family member? Is zillow just inaccurate about something?

This is southern california, so there is no way these could be open market sales. Pasted details from 5 examples:

Sale History
11/07/2006: $20,100
06/25/2003: $353,500
09/25/2001: $273,000
08/16/1995: $147,500
07/06/1994: $156,000

2006 Property Tax $5,107
Total assessed value: = $374,646
Assessed value bldgs: $114,256
Assessed value land: + $260,390

Sale History
05/25/2006: $90,909
06/24/2004: $417,000
12/29/2000: $190,000
10/28/1998: $147,000
06/11/1993: $138,000
2006 Property Tax $4,568
Total assessed value: = $433,846
Assessed value bldgs: $71,319
Assessed value land: + $362,527

Sale History
08/17/2006: $17,000
03/23/2005: $835,000
12/07/2001: $480,000
12/06/1999: $402,000
06/24/1993: $300,000
2006 Property Tax $8,889
Total assessed value: = $851,700
Assessed value bldgs: $243,971
Assessed value land: + $607,729

Sale History
12/14/2006: $78,509
06/10/2005: $657,000
05/29/2003: $440,000
2006 Property Tax $7,968
Total assessed value: = $670,140
Assessed value bldgs: $264,873
Assessed value land: + $405,267

Sale History
09/08/2006: $73,500
09/02/2004: $480,000
2006 Property Tax $5,520
Total assessed value: = $488,580
Assessed value bldgs: $107,589
Assessed value land: + $380,991