WebP that got lazy on the way out
Plenty of WebP files in the wild were converted quickly: default settings, no per-image tuning, sometimes from an already-lossy source. Modern WebP encoding does much better when it’s allowed to try.
That’s the whole job here: re-encode each file at full effort, compare the result with the original, keep it only if it’s genuinely better. Moving WebP files keep their frames and timing.