OK, time for me to be Mr. Unpopular and motivate some people to complain to the site administrator for being too negative.
The numbering machine used to print serial numbers on notes is quite a dynamic instrument. It has 90 groups of reels so that each note on a 45-note sheet gets two numbers. Those reels can be adjusted to print numbers anywhere you want them, so they can be moved up, down, and side-to-side. I don't know how easy it is to move all those "heads" of reels and make adjustments, and how well they stay in place during a print run. They probably have to be recalibrated now and then. When the two serial numbers used to print one note are not in perfect vertical alignment, it could be due to one head moving during a print run, or they might have been set that way by mistake. Machine operators are not perfect.
I have found quite a few notes with slight vertical offsets between the two serial numbers. I didn't save any because these offsets did not put one of the serial numbers outside of the designated space where they are found. Sheets of paper fed into the machine don't always line up perfectly either, so the result is the serial numbers can be found with quite a variability in placement within their designated area. Also, I have noticed that the horizontal distance between the two serial numbers can vary quite a bit from note to note. I observed this years ago with the Journey notes; I have not checked if this is still happening on polymer notes. Again, it could be operator error or due to slight movements of the printing heads laterally. Changes in lateral spacing don't have the same visual impact as changes in vertical spacing since serial numbers are oblong horizontally.
When single digit offsets occur, it is the result of one reel on a printing head being out of alignment. When I hear people say that this is not an error, I think they are responding partly to the way these notes are being sold on auction sites. I don't dispute that these single digit shifts are common on current polymer notes, and Internet auction sites are full of common notes. If someone had been listing numerous notes with shifted serial numbers on eBay, would popular opinion of these notes also turn sour? Would it depend on who was selling them and what kind of sales pitch they were making?
Obviously, visual impact correlates well with the value of error notes. For those notes with lesser visual impact, personal opinion plays a bigger role in determining value, and personal opinion is quite frivolous at best. Look at coins that have dot errors. Lots of coins have dots resulting from die chips and other wear. But the right dot on the right coin occasionally creates very popular varieties, and collectors will sometimes pay ridiculous prices to get one.