I don't think it is a simple as you suggest, the mystery is not solved.
On real books, with a long toc and setting the start to the first chapter, just after the toc, I have seen several different results, with different but similar books, with a K3 (in all cases correct in the Previewer):
1. Works perfectly.
2. Shows the bottom end of the toc.
3. Starts about 4 lines down on chapter 1.