Interactive Brokers currently shows the following data for SPX options at strike 3000 and expiry 2020-09-17:
- calls: bid/ask 234.10/236.30, theta -0.362
- puts: bid/ask 146.70/148.40, theta -0.225
Then for E-mini future options with the same strike and expiring just one day later (2020-09-18, for both the option and the underlying future) the following:
- calls: bid/ask 234.25/236.75, theta -0.279
- puts: bid/ask 146.75/148.50, theta -0.284
Why is theta -0.362 for SPX calls, but -0.279 for E-mini calls when they are practically the same intsrument?
Is this to do with the fact the former are Europen-style and the latter American-style? (If so, why would American options decay faster? How does this tie in with the fact that the prices are almost the same and presumably still will be tomorrow?)