URL contains a number after node/nid: why?
Drupal's path-matching is greedy. The path node/%nid will match anything with that string at the start, that has a separator after it. e.g


node/%nid/1
node/%nid/giraffe
node/%nid/barack-obama
node/%nid/this/is/just/too/much


The 'extra' path segments are passed on as extra parameters to the item's page callback, whether it wants/uses them or not.

The page that you see at node/%nid/1 isn't different at all from the standard node/%nid page. 

You'd do well to keep those URLs out of the page source at all costs, or make sure you have an appropriate canonical URL set, as Google will consider them duplicate content otherwise.