I couldn't help noticing that your last file (junk2.16) appears to have an earlier modification time than the others.
I hadn't noticed that. That probably does (at least partially) explain it. The last file does appear to be a left over from an earlier run.
I say partially, because I'm having great difficulty coming up with an explaination for how a left-over file from an earlier run could combine with a full set from a later run and recombine to reform a file that diffs equal to the original? Or how that combination of filesizes & numbers could be arrived at.
I cannot reproduce the results, although (I'd swear) I have seen this happen at least twice.
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
The "good enough" maybe good enough for the now, and perfection maybe unobtainable, but that should not preclude us from striving for perfection, when time, circumstance or desire allow.