Canceling machines of this era were fed a stack of manually faced mail. The stack was pushed against a "separator" mechanism of a reverse-rotating wheel which is adjusted to allow for average-thickness letters to pass one at a time. Here is the illustration from the operators manual for an International Model HD-2 machine (cranked by hand, but still the same principle as the high-speed machine your letter passed through, only slower):
The separator mechanism is spring loaded to allow thicker letters to pass without jamming, but thinner letters and postal cards often would double-feed resulting in total or partial skips. In your case you have incomplete separation resulting in a partial skip, likely a case of minimal contents allowing a second envelope to slip along, both getting only part of the cancel. Consider this partial overlap:
Looking like this when spread apart:
Then your cover being fed back through again and getting the full cancel added here in red:
The other cover (the orange paper sketch) was probably not re-fed as it had a canceled stamp and a full date.