WebP is an image format made for websites, and it comes in two kinds that trace differently. Lossless WebP keeps every pixel, the way PNG does. Lossy WebP throws detail away to save space, the way JPG does, and leaves faint extra shades along edges. A WebP saved from a website can be either kind, and the file name won't tell you which.
A lossless WebP has the same pixels as the PNG it was made from, so expect what PNG gives: flat colors come back close to the colors they were drawn in. A lossy WebP behaves more like a JPG. Expect extra near-duplicate colors, which Reduce colors in the editor can merge into a smaller palette.
The PNG to SVG and JPG to SVG pages show both cases with real color counts.