When you buy the PiL book you are also contributing significantly to
the project. (Actually, you are contributing more than when you buy the
manual, since the book is more expensive ;)
However, we like to think that people buy PiL because they really want
to read such a marvelous book, not because they want to help Lua :)
The manual, on the other hand, is a reference work. We don't think
people will buy it to read it from cover to cover. Actually, most people
prefer to consult reference works online, not on paper. Those people
would not buy the manual per se, but they may buy it if they know they
are helping the project.
So, the difference is mainly about marketing strategy ;)
