It is measured against the estimate, not against last year, so a company can grow profit strongly and still deliver a negative surprise. Index-level surprise figures are averages, which means one or two very large gains can lift the whole number without the typical company doing anything unusual.
Awaiting gradingThe S&P 500's Record 15.7% Margin Drops to 14.4% Without AlphabetCheck after Microsoft, Meta, Amazon and Apple report Q2 results whether their margins improved similarly, that determines whether 15.7% reflects a broad AI boom or an Alphabet-sized illusion.