now i know more about samba, and somewhat insane defaults:

The DOS archive bit is set when a file has been modified since its last backup. One motivation for this option is to keep Samba/your PC from making any file it touches from becoming executable under UNIX. This can be quite annoying for shared source code, documents, etc...

Note that this requires the create mask parameter to be set such that owner execute bit is not masked out (i.e. it must include 100). See
           the parameter create mask for details.

           Default: map archive = yes


So, by default config, the act of editing a file on a windows client (which sets the archive bit) makes that file executable.

The full set of samba directory attributes are now:
  read only = no
   writable = yes
   guest ok = no
   guest only = no
   create mask = 0744
   directory mask = 0777
   force directory mask = 0555
   map archive = no