Why a different viewport at the same resolutions and the same device diagonals and how to calculate it?

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Hello, forum users! I can’t understand where is the fine line between the viewport of different devices with the same screens? How is the viewport formed in general and why (if clumsy), then the width / height of some devices must be divided by 2 - (720 x 1440=360 x 720) to find out which viewport, and others by 3 (1440 x 2560=360 x 640), and the third by 4 (1440 x 2960 = 360 x 640). Coincidentally, it turns out 360 everywhere, but not always. How to find the thin edge of a viewport and how can a viewport be different with the same characteristics?

Device #1: Nexus 6P, 1440 x 2560 resolution, 5.7-inch screen. Viewport 412 x 732
Device #2: Samsung Galaxy Note 5, 1440 x 2560 resolution, 5.7-inch screen. Viewport 480 x 853