Retailer Numbering Off

I have noticed that when I delete all the retailers of a book and replace them with new links, the numbering is off. There are two number sequences that arise:

1, 2, 4, 5, 6, 7

or

1, 3, 4, 5, 6, 7

I haven’t gone higher than 7, and nor have I noticed any bugs arise from this strange phenomenon.