Why does ADB sideload always stop at 47%?

The answer can be found on Reddit

If I remember correctly (from a discussion long ago), Google's ROMs are transferred about 2.2 times across the sideload. To make the progress bar add up to 100%, they said "2.2 times the file size equals 100%". Our ZIPs are transferred just once, so the final progress is at about 47% with a success. The xfer seems to show the transferred data compared to the file size. Ours clocks in at x1.00 or x1.01, while Google's is about x2.something.