NetScaler ECV monitors not only check if the server is up, but also check if the requested content is present on the website. If you are requesting "200" as a response in ECV monitor, then the "200" must be present in HTML body on the website. HTML is returned in HTTP message body. 

HTTP-ECV and HTTPS-ECV monitors check if web server is up by checking the requested content is present on the web server.
HTTP-ECV Monitors will search for the text string defined in the parameters.

For example, a monitor might be defined to probe a web server that is a front end to a database, and the HTTP response should be checked to ensure that the web server is querying the database correctly.

The HTTP-ECV monitor uses the following parameters:
send [""] - HTTP data is sent to the service
recv [""] - the expected HTTP response data from the service

As an example, you can define a custom HTTP-ECV monitor as follows:
> add monitor <name> http-ecv –send “GET /siteup.html” –recv “site_is_up”

This monitor causes a NetScaler appliance to probe the back end HTTP service that is bound to a monitor with a HTTP GET request for the file %wwwroot%/siteup.html. The appliance expects to receive a response containing the string “site_is_up”.

Additional Resources

CTX120921 - How to Configure HTTP-ECV Health Monitor for Internet Proxy Servers
CTX205406 - How to use HTTPS-ECV health check monitor on NetScaler