Settlement is calculated from actual opening trade prices in a strip of index options, not from the index's own published level. That makes it a real, tradable print — and one that can differ noticeably from where the index was quoted moments before.
The distinction traps people who hold volatility options into expiration. Your contract does not settle against the index value on the screen; it settles against a number produced by an auction in a different instrument, and the two have historically diverged by several points on stressful mornings.
Example: the volatility index closes Tuesday at 21.4 and futures at 21.6. Wednesday's opening auction produces a settlement of 23.1 because index puts open wide. Every expiring contract settles to 23.1, and anyone who modelled the risk from the 21.4 screen print was looking at the wrong number.
Related: volatility-futures, vix-options, am-settlement, exercise-settlement-value