GPS-enabled devices are typically accurate to within a 16-ft radius, but not all phones have the same level of accuracy. As a result, the exact location of your clue will appear to vary, when in fact it is your position that is varying based on your phone’s ability to determine your current location. The accuracy depends entirely on your phone’s capabilities and the GPS satellite's ability to determine your actual location.
This same phenomenon can be observed when using a Google map to find your current position. Your location appears to float and changes because your phone is “listening” for signals from navigation satellites circling the earth. This is normal and does not indicate any problem with the Cryptic Quests system. It even makes the game more fun, especially when playing with a friend, because you can sometimes “find” the exact same clue several feet apart from each other.