Why does the historical zestimate graph not reflect accurate past zestimates.

I know for a fact that the zestimate on my home was over $300,000 for a short time during 2007.  However, now in 2012 if I look at the graph of past zestimates, it does not show the zestimate ever reaching over $300,000, rather it shows $279,000 as the highest historical value.  This is very very disconcerting to me.  It appears that zillow has retroactively changed the zestimate, but I find that hard to believe.  Maybe there is another explanation.  If there is retroactive changes to zestimates, it undermines the integrity of the zestimate.  (And this is in Oregon...zillow automatically filled in staten island for location, despite saying it was an optional field...I don't know how to change it now that Q is submitted...can only edit Q)