Atypical ferrimagnetism in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Ni</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:msub> <mml:mi>Nb</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant

Ferrimagnetism typically emerges from chemically distinct magnetic ions or the same element at two inequivalent crystallographic sites, rendering unequal moments. In contrast, ${\mathrm{Ni}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ has been recently discovered to show a different mechanism, where identical ${\mathrm{Ni}}^{2+}$ ions with the same ligand coordination develop unequal magnetic moments purely due to differences in local environments. Here, we investigate the microscopic origin of this emergent mechanism through a synergy of powder neutron diffraction, inelastic neutron scattering, and first-principle-based calculations. We demonstrate that the ${\mathrm{Ni}}_{A}$ and ${\mathrm{Ni}}_{B}$ sublattices, while sharing the same nominal valence, differ in their magnetic dimensionality: ${\mathrm{Ni}}_{A}$ forms quasi-one-dimensional chains with enhanced $p\text{\ensuremath{-}}d$ hybridization and a reduced magnetic moment, whereas ${\mathrm{Ni}}_{B}$ retains a nearly two-dimensional geometry and a full $S=$ 1 moment. Our results underscore the pivotal role of spin dimensionality and local structural distortions in stabilizing ferrimagnetism in systems with electronically equivalent magnetic ions.