The relativity of simultaneity is one of the fundamental consequences of Lorentzian spacetime. Nevertheless, discussions of moving clocks and light propagation are frequently interpreted through the longitudinal quantities and , creating the impression that relativistic effects can be understood entirely through one-dimensional propagation. This paper argues that such quantities are not the mathematical origin of simultaneity shifts. Instead, the origin of relativity of simultaneity resides in the synchronization structure encoded in the spacetime transformation itself. The synchronization term naturally defines a transport law whose closed-loop transport generates a measurable holonomy. The Sagnac effect emerges as the experimental manifestation of this synchronization holonomy. The analysis is then extended to the Causal Lorentzian Theory (CLT), where the transverse coordinates participate explicitly in the transformation. The resulting framework reveals a direct relationship between simultaneity, proper time, synchronization transport, transverse geometry, and closed-loop effects.