I have seen this several times, for example, this post. At the moment of asking this there are only 6 visible comments, but the total number says it's 7. Why is that?
It's probably because of removed comment(s). Among possible reasons are: removal by Reddit's site-wide automated spam filters, or if a user is shadowbanned in Reddit.
A shadowban is the tool we currently use to ban people when they are caught breaking a rule. It causes their submitted content and user profile page to be visible only to themselves while logged in. Moderators can see their comments within their subreddit (since they can see "removed" comments in the subreddit they moderate), but no other users can see their content, and nobody else can see their userpage.
Moderators of a subreddit can also remove comments within the subreddit they moderate. They can also automate removal of comments, including all comments by a specific user through AutoModerator.
From Reddit's FAQ page:
There are missing comments in a thread
If you find a thread that says there are more comments than you can see in the thread, the "missing" comments were probably removed - by the moderators of that subreddit, by the reddit spam filter, or by the subreddit's local AutoModerator. A comment which is removed in this way actually stays in the thread; it is merely hidden so that readers can't see it. So, reddit counts a removed comment in the total comments for a thread, but doesn't display it.