I'm sure this is probably easily explained but can someone elaborate on why the low on spy yesterday was approx 110.10 with ES Mar10 low was approx 1091 but this morning the low on spy was 109.73 while the low on ES Mar10 low was 1093.25. It's like the ES Mar10 chart this morning was shifted up four or five dollars. ie they don't match or come close even. Shouldn't they behave the same? ie an open of the same price in ES MAR10 should result in an open of the same price in SPY (not an open of a lower price)? Is it because of options expiration and why? Wouldn't this just be an arbitrage opportunity? TIA!