The qualification fi bab al-din in the definition of Sunnah refers here to an approved religious way, not every personal or social habit.
For instance, someone's preferred way of resting does not by itself become an emphasized legal Sunnah. Conversely, acts with established religious significance and evidence of regular observance may be classified differently.
Outward resemblance alone is insufficient. Intention, legal evidence, the status of the practice as an established way, and its precise juristic classification all matter. Different technical uses of the word Sunnah should not be collapsed into a single category.