It has to do with how they clock or log latitude and longitude coordinates. The more it logs GPS the more accurate the app. So, for an example, it pings every second it collects less data vs collecting every half a second. It tracks the GPS changes. So if I am in spot A and spot A has Long/Lat of this, when I am on B it has Long/Lat this. It tracks the time from A-B and it gives your speed. So when it pings the GPS this second on A and the next second it pings and you are B, it calculates it took 1 second to go from A-B so this is your speed. Now take the same example but use the half a second ping and it is way more accurate. So you ask why are the apps not all using the half second vs the 1 second? Battery. For a true accurate reading it would have to be a constant ping and that would kill your phone very quickly on battery.
Edit: I also forgot to add new or old phone or even what kind of phone. Older phones use older GPS chips. It boils down to how accurate (with-in feet) the GPS chip tracks. Older phones (or older chips) have less accurate tracking. You hear some GPS can find something within 10 feet. That would be less accurate of one that could track to 1 foot. Some are less some are more. The less distance of tracking the more accurate.
Last edited by kungfulu; 04-05-2013 at 02:09 PM.