Confusing file permissions issues when uploading wordpress plugins

I'm on my own Ubuntu box on AWS. I have two users: medsyd (the owner of my wordpress site) and www-data (which apache runs as). For now, to try get things working, I've added both to each other's groups - so www-data belongs to the medsyd group, and medsyd belongs to the www-data group.

I'm trying to make it so that wordpress can upload/install plugins without asking for FTP access. I can upload images/media just fine - but plugins seem to be behaving really weirdly and not following what I would expect given the file permissions.

Here's what I've tried:

1. medsyd:medsyd as the owner of the site directory and all it's contents; 775 directories, 664 files. Wordpress asks for FTP. Why? (the www-data user should be able to write/read the correct files since it belongs to the medsyd group)

2. medsyd:www-data as the owner of the site directory and all it's contents; 775 directories, 664 files. Wordpress asks for FTP. Why? (the www-data user should be able to write to read/write the correct files, since the files all belong to the www-data group)

3. www-data:www-data as the owner of the site directory and all it's contents; 775 directories, 664 files. Wordpress does NOT ask for FTP. YAY. But www-data shouldn't own all my files.

4. www-data:www-data as the owner of the wp-content directory (and all it's contents) ONLY (everything else outside wp-content owned by medsyd:medsyd); 775 directories, 664 files. Wordpress ASKS for FTP. (Why does this differ from the example just above, which worked, where www-data owns everything outside the wp-content directory aswell?)

What logic does wordpress use to decide whether or not to ask for FTP when installing plugins? I can't seem to think of any that would reconcile with the 4 examples above.

Edit: I understand there are other ways around this - eg changing the user apache runs as, storing FTP credentials in wp-config etc, but regardless, I really what to understand why the permissions I'm using above aren't behaving as expected.

via Chebli Mohamed