Hosting 2 apps on different ports using nginx proxy_pass

Why does the proxy_pass http://backends/; under the location /textcat bring me to the root of the server (localhost:8081) and proxy_pass http://backends; (no trailing /) brings me to (seemingly) localhost:8081/textcat.

I am trying to host two applications running locally on the server. These two applications are running on port 8081 and 8082. I would like to access the app at 8081 via the /textcat location and 8082 via /ner. I was able to do this with the configuration below. But I dont quite understand why this works?

upstream backends {
    server localhost:8081; # change to the port the webapp is listening on.
}


upstream backends_NER {
    server localhost:8082; # change to the port the webapp is listening on.
}

server {
    listen 443 ssl;
    server_name "";
    ...SSL Stuff...

    location = /textcat {
        proxy_pass http://backends/;

  }
    location /ner {
        proxy_pass http://backends_NER/;# This seems to work because of the trailing slash. Since the http_referrer is /ner our proxy path has to have the trailing /
  }
# have to redirect on a any location request, and if the referrer is /textcat do localhost:8081
    location / {
    if ($http_referer ~* (/textcat) ) {
        proxy_pass http://localhost:8081;
    }
    if ($http_referer ~* (/ner) ) {
        proxy_pass http://localhost:8082;
    }
  }
}