With the zero setting, and the fix not moving around constantly, the
accuracy of GPS is most dependent on having a clear view of the majority of
the satellites with minimal multipath propagation caused by high buildings,
bridges, mountains, towers and anything else reflecting the incoming
signal.

In a clear parking lot, with more than 6 satellites in direct view, it will
place the GPS in its location in a single parking space with no multipath
in flat country.  In city canyons of high rise buildings where the actual
direct view of the sky is only a few degrees in most directions, you're
lucky if it stops going crazy in the area of the whole aforementioned
parking lot at the mall.  It'll be all over the place as you move around,
even walking and really screwing it with multipath going in a dozen
directions at once.

GPS is very dependent on TIMING, the time it takes the signal to get from
the precisely transmitting bird to your receiver's fixed delay.
http://electronics.howstuffworks.com/gps.htm
This website does a great job, in a series of sections, to show how GPS
works and that TIMING is EVERYTHING in the GPS system.  The text is nicely
detailed without becoming the math monster of spherical trig the scientific
websites quickly become.  Just take the calculations for granted.

It's really quite neat how the cheap GPS gets an atomic reference clock for
free (displaying it on your screen, quite handily).

To really see what TIMING means to your GPS accuracy with awful multipath
making the paths from the birds go crazy, just take your GPS mapping box
into an open restaurant with a metal roof to block out the direct signals
and large windows on at least 2 sides so the signals can bounce around off
the traffic, buildings, etc., the GPS can see through the window.  Move
away from the windows far enough so you cannot see the sky from your table.  
Sit, eat, and watch the crazy patterns of a system gone mad.....