Hi,
The reason is because the google maps web page works differently than the google maps API. For the API to work it must find an exact match, but the google maps page is different, it is like a search engine, if it can't find the exact location it brings back a list of approximate matches. These matches may contain the correct location but they will have the address listed correctly/differently than you entered it in mojoPortal, you can copy the address format from the google maps search result and use that as the location and then it should work.
Its a good question and it comes up from time to time, so I have added it to the FAQ page.
Best,
Joe