E.g. about 5% through the file data, the progress bar shows ~40%

It races ahead during the first file, and then consumes the remaining space in approximately correct proportion. "... Remaining" shows similar inaccuracy.
Here's a capture of a different comparison showing the same effect at approx 15% of file space:

It races ahead during the first file, and then consumes the remaining space in approximately correct proportion. "... Remaining" shows similar inaccuracy.
Here's a capture of a different comparison showing the same effect at approx 15% of file space:

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