Drop any mix of pictures. Photographs are written as lossy WebP searched to matched quality; screenshots, logos and line art as lossless WebP. Each row shows the real saving against the file you dropped — or that there is none.
Decided per file, measured per file
The WebP encoder is chosen for each picture from two measurements — how many distinct colours it has and how much of it sits on hard edges. Drawn pictures get lossless WebP from this page's own encoder; photographs get the browser's lossy encoder, searched until the result measures visually identical to the source.
The saving is never an average quoted from elsewhere: it is the difference between your file's bytes and the WebP actually written. When the WebP is not smaller — a well-compressed JPEG sometimes is not beaten — the page says so, and Best per picture would keep your original.