Despite sharing a common lattice structure, monolayer ${\mathrm{M}}_2{\mathrm{X}}_2$ compounds realize quantum anomalous Hall phases with distinct Chern numbers, a striking phenomenon that has not been fully explored. Combining first-principles calculations with symmetry analysis and tight-binding models, we identify two generic band-inversion mechanisms governed by the orbital composition and symmetry representations of $3d$ states near the Fermi level. When $d_{xz}$ and $d_{yz}$ orbitals dominate, a doubly degenerate $\mathrm{\Gamma }$-point inversion yields $C=1$; otherwise, inversions occur along $\mathrm{\Gamma }\text{−}\mathrm{X}$ and $\mathrm{\Gamma }\text{−}\mathrm{Y}$ at four $C_4$-related momenta, whose Berry-curvature contributions add to give $C=2$, distinct from scenarios relying on multiple band inversions at a single $\mathbf{k}$ point. The same mechanism consistently explains related two-dimensional systems, including LiFeSe, KTiSb, MgFeP, and Janus ${\mathrm{M}}_2{\mathrm{X}}_2$ derivatives. The mechanism provides practical guidance for screening and engineering tunable high-Chern-number insulators.

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• Chern insulators
• Monolayer films
• Topological materials
• Transition metal dichalcogenides
• Density functional theory
• First-principles calculations