Custom error 404 - bypassed in specific case

Question

I have a question regarding HTTP 404 error handling by IIS. I know that this is a subject discussed very much but still i cannot find the answer.

I have my custom page NotFound404.aspx and works just fine. On page not found the my url is */NotFound404.aspx?aspxerrorpath=/index_fake.aspx*

My question is: what happens if i edit this URL and at this moment i write */NotFound**XX**404.aspx?aspxerrorpath=/index_fake.aspx* ?

As i see, i get the default :HTTP 404. page in iis.. I don't understand what is happening? why in this case it passes over the application level and gets the Server default error page?

Tnaks a lot. Eduard