Where is your phone during your bike ride? GPS satellites are very far away so their signal is weak and the accuracy depends on the receiver getting a direct line-of-sight signal. Your body (or anything else that contains water or conducts) will block much of the signal so reception won't be nearly as good if you keep the phone in a pocket vs. on a handlebar mount. Accuracy will also be less in areas with tall buildings, lots of tree cover, or in canyons.