So, what are we talking here - about 6,500 steps? Thus, a 15% discrepancy?
Where was your S8+? Front pocket? Rear pocket?
What kind of watch? What step-counting technology does it use? (yes, there are different ways watches use to count steps)
Was your "hike" really a "walk" during which your arms swung freely and relatively steadily at your sides?
I might be inclined to think that your watch has the more accurate number but, consider that the typical adult has a step length of about 30 - 32" and thus the device that comes closer to 6,500 steps for your 3.1 mile hike may be the more accurate one.