I think it's likely because Topps is switching out which backs are in each wave. So Old Mill is in Waves 1, 3, and 4 but replaced with American Beauty in Wave 2. And with 10 waves this year it's possible it'll happen again. Also a big pain in the but is Topps released checklist had the cards numbered, but they are unnumbered like the original T206 set. NNO sets get sorted alphabetically so adding wave after wave will have a set out of order, but can be reordered by admin. So likely this will only happen once all of the set has been released. But jimetal7212 is right it's just noone has tried to tackle the set yet. I know I was waiting to add anything to make sure the cards were unnumbered again like last year.
If I can carve some time out tonight I'll look at both the 206 set and Keith Shore 89 bowman sets and add checklists, unless someone beats me to it.