28 and 29 have very old and suspect values. Oddly, 30 and 31 don’t seem as out of line but they dont fit the test of the series either.
There’s just not a lot of sales data points for those books. I definitely recommend you add grades to the books in your collection, or the values you will get will most likely not be applicable to your collection.
We were able to find a few more recent sales on those issues, but not a lot.