Probably because the first instance you start Maps, it would rely on the best location fix available at that moment which is radio triangulation (WIFI, cellular data). And triangulation is not accurate and sometimes really off depending if you're in the center of 3 or more base station coverage. In other words, its really a best guess location fix. However, wait longer and when the GPS get its fix, it will show you a more accurate positioning. We do know the GPS on phones are slow to get a location fix, the same applies for iPhone 3G. Radio triangulation only gives you the impression of a 'faster response' and its meant to supplement the GPS when it is not getting a location fix yet.

This post has been edited by davidmak: Jun 9 2009, 09:37 AM