I don't know specifically differences between apps... but I do know that even on the web, AppMeasurement has logic to switch from GET to POST based on the length of the server call... If the data will result in more than 2000 characters the method will switch to POST. It may be just that some of the device specific information that is included on those calls is just pushing the one over the threshold...
I have never noticed any difference in tracking via GET or POST, if that puts your mind at ease.