When I last helped out at the LHS in 2016, we starting noticed this also. Up to that point the 5 digits after 23922 matched the Lionel sku number. Now some match, others don't. For the ones that don't match, there did not appear to be any logic to the numbers used, other than avoiding duplicate numbers.
Looking at the photo of the American Flyer box 1921221, the most likely reason the bar code doesn't contain 21221 is that this number was already used (6-21221 K-Line Milwaukee Road S2 diesel). See image below.
To further confuse matters, the same number on the barcode may not be a unique item, as looking quickly online found the barcode from Lionel item 6-19132 which has exactly the same digits as the American Flyer box 1919132.
