Because index members do not open at the same instant, the SOQ is assembled from each stock's first trade rather than from a single index snapshot. The resulting number often differs from any level the live index prints that morning.
The mechanics create real order flow: arbitrageurs holding futures against baskets must trade the underlying stocks on the opening auction, which is why volume explodes at the open on expiry days.
Example: if the S&P index quotes 5,020 at 9:31 but the SOQ computes to 5,024.61, every expiring ES contract settles at 5,024.61 — a 4.6-point, $230 per contract difference from the screen.
Related: final-settlement, cash-settlement, es