Can failure to increase new user's UID consistently by one be related to this line inside user.module file?
uid and other identifiers are not sequential since Drupal 7. Identifiers are generated using a sequence API, see this issue: https://www.drupal.org/node/356074

If you check the db_next_id you'll see that the number passed as argument is just de minimum id that is considered valid.

See also this question and answer: What could cause uids to not be numbered consecutively?