Why does shouldStartLoadWithRequest not load my /amp version in the webview, and does in safari?

I have an iOS app with a UIWebView in it. In my webiste all my pages have an /amp version in it ( i.e www.website-example.com/pageX/amp )

In the shouldStartLoadWithRequest: method I do :url = [url URLByAppendingPathComponent:@"amp"]

if I do [[UIApplication sharedApplication] openURL:url];return false; (so it opens the url in Safari) it opens the good /amp version.

Otherwise, if I return false (after appending /amp), it opens the url in the webview, it doesn't open the /amp version, although the URL contains /amp.

edit: I'm adding Swift tag, because it's not a programming language problem, it's understanding shouldStartLoadWithRequest method.

What is the problem?