I have iPhone 15 and apple watch series 9

The accuracy of iPhone step counting can vary depending on how you use your phone throughout the day. While iPhones use sophisticated accelerometers to detect steps, these sensors can be influenced by factors that differ from day to day.

Here's a breakdown of the reasons:

• Carrying position: How you carry your phone can significantly impact step count. If it's bouncing around in a loose pocket, it might register extra movement as steps. A secure armband or holster would provide a more consistent reading.
• Activity type: The iPhone can't distinguish between walking and other activities like jogging or using an elliptical trainer. These movements might not translate perfectly to step counts.
• Walking style: Our walking styles are unique. If you typically walk with short, quick strides one day and long, purposeful strides the next, the iPhone might undercount or overcount steps compared to your baseline.

Ultimately, replicating every detail of your daily routine is nearly impossible. These slight variations can lead to some discrepancies in daily step measurements.

Frankly, I never tested a calculator or any electronic measuring equipment for its correctness, be it my Thermometer, BP monitor, Blood Sugar Monitor or Oximeter. It never occurred to me.

Measuring and validating steps in percentage is beyond my imagination. I always use them as a guiding tool in my day to day workout. If at all an error exists, it will be averaged out over a period and never need to know the exact steps taken or recorded by my iPhone on a particular date. Whether it is 8000 steps or 10000.

Unless you are using it in a laboratory for some experiments. If yes, in that case you have chosen a wrong equipment for your science experiments.