If you are otherwise feeling OK with no inappropriate dizziness (a little dizziness when standing up is normal), no chest pain, no shortness of breath, you function well at work/school and during exercise, my best advice is to leave your pulse/heart rate alone and not measure it.
The heart controls itself, and the heart rate is supposed to vary. Your heart is not a metronome, some heart rate variation, occational "skipped" beats or double beats are all common.
If you are worried, you can visit your doctor and register an EKG, but again, if you are feeling well and healthy, there's really no need to rush it.
It could be that the phone is not counting correctly. Try to count your pulse by feeling the beats on your wrist and see if that is also giving these varying results.
How are you taking. Taking in same body body position?