You have Measure 47 (1996) to thank.
http://en.wikipedia.org/wiki/Oregon_Ballot_Measure_47_(1996)
Market Values and Taxable (Assessed) Values are 2 different numbers.
Taxable Values are limited to a 3% increase per year (Voter approved Bond measures exempted, which is why they are listed separately).
If the Market Values take off at rates far higher than 3% per year, as they did during the bubble, the 3% cap keeps the tax bill from similarly going up.
The downside, is that when the Market Values decline, the property tax bill will continue to increase 3% per year until the Market and Taxable Values are the same.