The file man/commands.texi distributed with GNU Emacs version version 19.25 contains, near the top of the file:
@c See file emacs.texi for copying conditions. @iftex @chapter Characters, Keys and Commands This chapter explains the character set used by Emacs for input commands and for the contents of files, and also explains the concepts of @dfn{keys} and @dfn{commands} which are necessary for understanding how your keyboard input is understood by Emacs. @end iftex @node User Input, Keys, Screen, Top @section Keyboard Input
Texi2www doesn't see the @chapter since it's inside an @iftex block; this confuses texi2www's chapter numbering. My fix was to change this section to:
@c See file emacs.texi for copying conditions. @node User Input, Keys, Screen, Top @chapter Characters, Keys and Commands @iftex This chapter explains the character set used by Emacs for input commands and for the contents of files, and also explains the concepts of @dfn{keys} and @dfn{commands} which are necessary for understanding how your keyboard input is understood by Emacs. @end iftex @section Keyboard Input
killing.texi, misc.texi, and trouble.texi have similar problems.