Unable to chgrp or chown file from unicorn - ruby-on-rails

Question

I am running into an odd issue trying to chown or chgrp a file from a unicorn process. Running the same code from rails c, it changes the group to the correct group, ex:
bash-$ whoami
zac
bash-$ groups
zachallett sysadmin
bash-$ ls -la
...
-rwxrw---- zac sysadmin 154 Nov 1 15:33 file.txt
...
rails controller action:
def controller
file = "#{Rails.root}/file.txt"
%x(chgrp zachallett #{file})
end
in unicorn log:
chgrp: changing group of `/var/www/app/current/file.txt': Operation not permitted
output of ps aux | grep unicorn:
zac 6579 0.0 1.1 254640 45188 ? Sl 17:13 0:01 unicorn_rails master -c config/unicorn.rb -E production -D
zac 6582 0.0 1.0 254640 42704 ? Sl 17:13 0:00 unicorn_rails worker[0] -c config/unicorn.rb -E production -D
zac 6585 0.0 1.0 254640 42704 ? Sl 17:13 0:00 unicorn_rails worker[1] -c config/unicorn.rb -E production -D
zac 6588 0.0 1.0 254640 42704 ? Sl 17:13 0:00 unicorn_rails worker[2] -c config/unicorn.rb -E production -D
zac 6591 0.0 1.0 254640 42704 ? Sl 17:13 0:00 unicorn_rails worker[3] -c config/unicorn.rb -E production -D
zac 6594 0.0 1.1 254728 45004 ? Sl 17:13 0:00 unicorn_rails worker[4] -c config/unicorn.rb -E production -D
zac 6597 0.0 1.1 254728 45072 ? Sl 17:13 0:00 unicorn_rails worker[5] -c config/unicorn.rb -E production -D
zac 7274 0.0 0.0 103232 848 pts/0 S+ 17:32 0:00 grep unicorn
Running the same chgrp from rails c, it changes the group just fine. So user zac owns the file, and is part of the sysadmin group, however I am unable to run chgrp on the file from a unicorn process.
EDIT: Add unicorn.rb config file
env = ENV["RAILS_ENV"] || "development"
working_directory "/var/www/<APP>/current"
pid "/var/www/<APP>/shared/pids/unicorn.pid"
stderr_path "/var/www/<APP>/shared/log/unicorn/stderr.log"
stdout_path "/var/www/<APP>/shared/log/unicorn/stdout.log"
listen "/var/www/<APP>/shared/sockets/unicorn.socket"
worker_processes env == "production" ? 6 : 2
timeout 120
preload_app true
user "zac", "sysadmin"
before_fork do |server, worker|
old_pid = "/var/www/<APP>/shared/pids/unicorn.pid.oldbin"
if File.exists?(old_pid) && server.pid != old_pid
begin
Process.kill("QUIT", File.read(old_pid).to_i)
rescue Errno::ENOENT, Errno::ESRCH
# already killed
end
end
end