This is from my latest GreatCollections package; part of the box was these 3 proof coins, all certified PF 67 by ANACS: 1961 dime, 1962 dime, and 1961 quarter. I noticed this one detail about these. They are consecutive serial numbers, but then I saw this: The 1962 is the one coin between the 2 1961's! Just to think, we could've had a consecutively numbered 1961 proof set, but the 1962 had to stick it's anal area in the way. I betcha anyone on the subreddit r/MildlyInfuriating have never seen something so hyper-specific as this one little detail.
In my one experience submitting to ANACS at a show, they sticker the submitted flips (or slabs) with pre-printed sequential cert# labels in the order they're listed on the submission form. I would guess a mailed submission is about the same. So I think you can blame whoever filled out the form. All my 1947 Canadian denominations came back in sequential slabs, because they were listed that way on the form. (I hesitate to ask, but why does it matter?)