Your GPS accuracy can change by the second. In a city with hundreds of networks on a single street, that data starts to form a wifi map that works well when the maps app has no/weak GPS.
Your phone sends data back to the maps company, reporting locations of wifi networks detected. My understanding is that it's not about networks you join, just networks your phone picks up on. Important note: Wifi assisted GPS is worth looking up. The Apple API tells iOS apps about signal strength using a 1-5 rating. Some apps increase the radius circle to reflect the strength/weakness of the combination of signal strengths. Now your phone actually knows how accurate it is. Not enough strong GPS satellite signals (in a tunnel, under a storm, etc) In that case, you'll continue to see wider variation, especially when signals are weak. However, most of us will be using a more rudimentary GPS chip for a while. The combination of those satellites and that cheap will bring precision to about a 1 foot.
Now Broadcomm is said to be launching dozen of new satellites with much higher precision but it requires a new chip. A weak signal can quickly reduce that to 100+ feet or even 1,000+ feet! If your phone is in position to receive 4+ strong satellite signals, most phones GPS should be accurate within 20 feet. Why is GPS better? GPS satellites use an atomic clock to send timestamped signals that are insanely accurate. However, it is supplemented by the other two when the GPS signal is weak. Even though it might seem like Wifi should be more accurate, GPS is best.
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