Just a guess.... Are you sure that you are comparing identical critters? If you were comparing two vendor graphs of the same option, I'd expect identical results. If you are comparing the average IV of a stock, either vendor may be including/excluding peripheral OTM options and that could skew the results.
Those IB charts are horrible... I set one up to see all the IV type data on INTC, changed the symbol to GOOG and it retained the IV data on INTC!! Why can't they do customer service and charts?? IB as an organization has almost engineered the human out of the picture including the end user......... I never use them without third party software to protect my psyche from their garish and clunky thingy... there has to be a market for software that would intercede on the user's behalf and make IB look/feel/work like Tradestation...