Question about D hirearchy and include directory

[bioinfornatics]

Dear,

Some projects seem to have an extra directory for their include 
directory, such as:
- stdx-allocator
- containers
- msgpack
- dparse
...

To draw the picture, I take as example the containers project and 
build using meson:

$ ls /usr/include/d/containers/containers/
cyclicbuffer.d  dynamicarray.d  hashmap.d  hashset.d  
immutablehashset.d  internal  openhashset.d  package.d  simdset.d 
  slist.d  treemap.d  ttree.d  unrolledlist.d

Why it is not : /usr/include/d/containers
Or why for (phobos/std) it is not /usr/include/d/std/std

Is it the expected behaviour or these meson.build should be fixed:
Example at line 94 
https://github.com/dlang-community/containers/blob/master/meson.build#L94
replace:
install_subdir('src/containers/', install_dir: 
'include/d/containers/')
to:
install_subdir('src/containers/', install_dir: 'include/d/')

Thanks

Have a good day