Different section numbering in HTML and PDF output

[Vladimír Marek]

Hello,

We generate html and pdf output from our source files and they have
different section numbering.
In HTML there is a section which is not present in PDF
in HTML:
1. About This Book
1.1. About the <our product>

in PDF the the Chapter numbering starts with:
Chapter 1 - About the <our product>

Any idea how this can be fixed?
We use sphinx-build 1.8.5

Thanks,
  Vladimir