Unable to accurately locate itself?

Why may my iPhone 4 be consistently off by 1/2 mile or so when trying to locate itself? I thought it was a software issue because Google Maps kept on trying to place me about 1/2 mile off where I was. Yet, I noticed that (Apple) Maps also did the same thing. Furthremore, now I discovered that when I used iCloud to try to locate my phone and I found it, it too located it about 1/2 away. (Granted in each case, the margin of error was about 1/2 mile radius; yet, that too is consistent.) I do not recall my iPhone being to much off in the past, and now it is a nuisance to use either Map as it attempts to reroute my path every mile I travel.