his Garmin step counter more accurate than iPhones fitness tracker?

I just have a general question as to how accurate the iPhones step counter is compared to Garmin‘s step counter. I have been using Garmin’s step counter for about six years and I recently compared it to my iPhones step counter and was surprised to see the results. My iPhone and my Garmin are on me 24/7. The iPhones step counter was off by about 6000 steps. I googled comparisons and numerous articles determined that the garmins pedometer is far more accurate according to studies that were conducted. Anyone has any insight on this?