As seen here: http://www.martinlugton.com/view-number-unique-visitors-page-set-pages-google-analytics/

You can see the number of unique visitors to your site as a whole in Google Analytics’ Audience report. But if you go to Behavior > Content Drilldown, to view a particular page of set of pages, you can only see “unique page views”.

Unique page views is a different measure to unique visitors. What a nuisance.

So the solution is to create an advanced segment in the Audience > Overview report, which includes the pages you want. (Inspiration taken from this comment by Rendy.S)

Let’s say I’m interested in the information section of the www.mind.org.uk website. All of this content lives inside the /information-support/ section.

So If I wanted to create a segment to look at unique visitors to this part of the site, I’d use a regular expression like this: /information-support.*
The . and the * at the end combine to mean “plus any number of any other characters”. i.e. this will match any URL that starts mind.org.uk/information-support

So if you ever want to view the number of unique visitors to a particular page or pages in your website, create a custom segment and user regular expressions to target the page(s) you’re interested in.