If you look at my Myzone Vs. Apple Watch measurements for Calories burned, you can see there is a difference of ~100 calories for the same activity. While this example is for calorie counts, I’ve also heard frustrations over mileage, and even weight/body fat measurements when comparing different devices.
When asking about the difference between devices, people question the VALIDITY of each device, when they should really be wondering about its RELIABILITY. In other words, does 3 miles on your watch always end at the same point in your run? If yes, then your distance counter is reliable & useful! Whether that is actually 3 miles or 3.1 miles means very little to most runners unless they are at the ultra elite level, and at that point they may use other ways to measure distance. If you are using it to compare measurements with your friends, I would suggest all using the same product to ensure reliability across devices- comparing apples to apples if you will. But if you’re trying to compare your one workout session to the next, just stick to the same method of measuring progress, as long as it’s easy to use.
But what about that 100 calorie difference? Honestly, whether or not your run is 300 or 400 calories means very little in most cases. The difference from 300 to 400 calories is pretty minimal, so I suggest just mentally taking an average of the two. Even better-use that average to lump the activity into low, middle or high intensity and base your recovery & food needs for the day on that.
Only if there are larger differences (let’s say ~300 calorie difference between watch & phone or more than a half mile as examples) I would look into the conductivity of the tool, and/or the accuracy of each device.
By the way- there are other reasons to check in with your calorie expenditures besides food allowances. Knowing today’s run was low to moderate intensity tells me sleep & mobility is less important tonight & I can push harder tomorrow without adding too much physical stress. I think that’s MUCH more useful info to know than whether or not I can have an extra slice of bread today!