Apache mod_rewrite redirects, but it's not asked to

Question

I currently have

RewriteEngine On
RewriteBase /

# RewriteRule index\.cgi/(.*) /$1 [R=301,L,NS]
RewriteRule . index.cgi [L]

I cannot understand why the active rule redirects everything from abc to index.cgi/abc, even though the R directive is not present.

This is supposed to produce an internal redirect.