Was that done on purpose?

I tried pushing Crystal Shell + Fine Grass.

I expected Rank 12 + 5 to be a max factor of 17. Since the explicit detail said 19 I got confused. With an Alchemy Book 3 on Superior, wouldn't it be 20 for +5 +3 = +8?

But without Alchemy books it would be +5 max with superior, right?

I just want to know if the calculator is still under revision or if there is a declarative that +7 is the "reasonably determined maximum" since no one would really spend money on trying to pull of a +8 with a book.