None taken! I find this odd, but can't yet put my finger on "I made a mistake" versus "I learned something new today", so welcome input!

First thing to check: When you calculated this, was this a snapshot of bid/ask and index level and SPY price at the exactly the same moment while the market was open?
The data was from today (East Coast U.S.) so both SPX and SPY at close Friday, data from Schwab.
BTW, I also saw this in older data I had lying around, I just thought I'd make the data snapshot for this post timely so pulled up Scwhab options chains.

SPY seems fine, makes sense, ask is always greater than IV.
The argument for why this holds for SPY is simple: if you held SPY shares, then you can buy one of those puts, and if Ask was less than intrinsic value (IV) then you could sell the shares using the put and make money, so that gets arb'd away.


The SPX "ask" in relation to "IV" surprised me.

In the post I floated the suggestion that because you can't hold (buy or sell) SPX shares that the arbitrage argument that works for SPY, where you can hold shares, doesn't work for SPX so SPX "ask" can float around in relation to IV.
But that makes my whiskers twitch.