The MVC RedirectToAction method simply sends the client (browser, in this case) a HTTP header called "Location" in the response, and sets the status code to 302 "Found", to indicate a suggested redirect. That header contains the URL of the view you specified, and puts any model data onto the querystring.
The client can choose whether to visit that URL or not. Browsers generally follow the link automatically, because it's user-friendly to do so. Other HTTP clients may not do so. That's how it works - this is general to the whole web, not just MVC. The redirect is always done as a GET, it's not possible to redirect via any other HTTP method.
If you want to just display the view without a redirect, then use
return View("addLeaveEntitlement", result.Model)
Then there's just one request, there's no redirect, and the URL does not change at all (not even to the regular URL of the "add" view - this may or may not be desirable).