Here is DIS chart with it's IV chart. Latest IV is 16.5% http://i66.tinypic.com/b7fk41.jpg Here is DIS option chain and i don't see anywhere this particular number 16.5%! http://i63.tinypic.com/167kxi0.jpg More to it, i bach tested some options with huge movement in IV chart and found funny thing. Sometime when IV chart is goign down -5% IV options IV is going up +0.75%.
That "ImpVolatility" lower study is a calculated average of different option chains, attempting to resolve to an effective "30 DTE" implied volatility value. (influenced by VIX lovers) Look at the two series surrounding 30 Days, and average with respect to time for 30 days, and the result should be very close. To get more insight as to how the temporal changes seem disjoint to underlying prices, read the CBOE VIX White paper on how VIX is calculated -- this can open your eyes (the ImpVol here should be a similar calculation)! Hope this helps.
Note, that if a couple of the adjacent near OTM strikes happen to have Zero BID, your ImpIV number can get erratic! Zero bid causes the strike to be ignored, two adjacent zero bids cancel out remaining OTM strikes from the equation. Is less likely on high liquidity options and indexes.
VIX calc is not that hard... simply put: It's the 30 day vol of SPX based on all options with 30 days till expiry with at least a bid. More weight is on the ATM... the further out of the money, the less weight given to it. When there is a 15 day and 45 day expiry... they average both option chains 50:50. If it's 20 days and 50 days... they average the chains 66:34. This way they calculate the 30 day vol. So I guess your IV chart in DIS is the way @stepandfetchit puts it... the average of a bunch of series...