Observation tolerance -- a scenario rule
Authored teaching scenario. Every observation, prediction and record below was written for this exercise. None was produced by running image software or by examining an image, and nothing here supports a claim about how any real tool or forensic technique behaves.

File size: within 15 KB.
width, height, chromaSubsampling, exif: must match exactly.

A scenario rule, not an empirically validated forensic tolerance. It is set for this exercise so you can apply a stated threshold consistently. A route is compatible on file size when its prediction is within 15 KB of the observed size. If more than one route satisfies that, file size does not distinguish them -- which is a fact about this evidence, not a claim that the files are identical.