See the option ‘Use external url for single events or locations’ in the EME settings, tab ‘Other’.
If this is not active, then #_EVENTPAGEURL returns the regular event url if no external url is given, and it returns the external url if one is given.
If this is active, #_EVENTPAGEURL always returns the regular url.
To always return the external url, use #_EVENTFIELD{event_url}, but I recommend to use an attribute if you don’t intend to use it as the real event url.