I never understood that.
I once talked to the captain of a boat (that sailed from the Mediterranean to the Caribbean islands) that in the straight of Gibraltar his GPS was inaccurate. He said that it was because of the satellites. But the GPS satellites don't know where you are; you're just receiving signals from them. AFAIK for military purposes the satellites introduce inaccuracies in the timing signals they broadcast, and only military GPS knows how to correct for those. But how is it possible to have a particular region on earth have inaccurate positioning.
Do you know how that works?
BTW, thanks Bill Clinton for removing (most?) of the inaccuracy and making GPS navigation in cars possible.