New users typically get assigned at gid 500. You can change this by modifying
UID_MIN in /etc/login.def
( Note: Some older distros started at 100, and some new distros made 1000 the default).
Users below the
UID_MIN are non system users (eg. apache, tomcat, ect..).
The author of the recipe likely piked a random number less than 500 and claimed it for their own purposes. While it is not best practice, it is unlikely another program is hard coded to use 403.
A better approach would have been for the recipe author to set
system = true at the user creation.
user 'td-agent' do
What are the dangers of creating a normal user with UID < 500?