Quote from spindr0:
If the two UL's track each other at an exact ratio of 10:1 and IV's are the same, the options should track pretty close to 10:1. IRL, they don't plus spread size varies. I assume there may be other small discrepancies as well. So I wouldn't expect an exact 10:1 correlation. But close.
6 or 7 to 1 is a bit extreme so like others have asked, got any examples?
Any descrepencies should be explained by dividends (spy is lumpy and spx is not), the extra day of optionality in spy, and any increase in funding requirements due to hedging with spy vs spx futures.
Except the dividends, the other two factors should be small and indistinguishable to any retail level trader (and most institutional level ones).
The dividends are real however, but they can only explain at most 60 cents/ 6 points in valuation (and that for short dated deep in the money call options).
I think the original guy made a mistake.