R CMD build installing the package to process help pages

Dear All,

If a package has dynamic manual pages (i.e. `\Sexpr{} macros), then R
CMD build installs the package, and (by default) creates the PDF
manual.

I understand that this is needed for build-time Sexprs. Could anyone
explain me why `R CMD  build` needs to install the package if all the
Sexprs are install-time and/or render-time? I must be missing
something here.

Thanks much,
Gabor