A pixel value is not an amount of light.

sRGB stores brightness on a curve, so level 128 — the middle of the number range — is 21.6% of the light, and 50% of the light is level 188. Sixty levels apart, a quarter of the whole range, and nothing in the number says so.

The engine is free and needs no account. Which matters because blending is arithmetic on those numbers. Mix black and white at half: interpolating the stored values gives #808080 at 21.4% of the light, and interpolating the light gives #bcbcbc at 50%. Sixty levels apart and 2.3× the light, from the same instruction. Both are real pipelines — CSS opacity and gradients interpolate the stored numbers, SVG filter primitives default to linearRGB — and nothing in CSS, SVG or a design file records which you meant. And because WCAG contrast is defined on light, a white panel at 18% over #1a1a1a is 9.86:1 (AAA) or 4.4:1 (AA large only) depending purely on which tool composited it. Every lane ships a worked example that costs nothing. Writing the report needs credits.