URL aliases and bidirectionality
By default in Drupal if you have node/7 with an alias of some/readable/path then a user can go to either of those URLs and it will load the page.

For SEO reasons it is considered bad to have the same page viewable at multiple URLs.

For this reason a very large number of drupal sites use the redirect module.

The redirect module forces users to always access a node from the same URL so if a user navigates to node/7 it will redirect them to the aliased URL instead.

It is likely the difference between the sites you are looking at is the redirect module.