Analysis of the values of tetragonality of the carbon martensite presented earlier in Ref. [1] is developed taking into account the factor of limited solubility of carbon in austenite, which causes the inhomogeneity of distribution of the local tetragonal distortions of the $\alpha$-phase lattice in the whole volume of the martensite crystal, and so the experimentally fixed value of tetragonality $c/a$ is the averaged value for the steel with a given content of carbon. A model of numerical estimation of the local tetragonality value of the $\alpha$-phase unit cell containing the C atom is proposed. As shown on the basis of analysis of the interatomic-distances’ changing in lattices of $\gamma$- and $\alpha$-phases as a result of the martensitic transformation, the location of carbon atom in the middle of edge $c$ of martensite b.c.c.-lattice unit cell causes to a strong local distortion of the lattice shape. As concluded, as a result, the martensite crystal of the carbon steel consists of mainly b.c.c. unit cells of $\alpha$-phase with rare (respectively to C concentration) inclusions of the ‘pseudo-cubic’ unit cells with one edge increased to 1.41$a$. Obviously, the aggregate of such enlarged carbon-containing edges of the ‘pseudo-cube’ oriented along the $c$ axis leads to a splitting of martensite diffraction lines {110} (that is most often used to determine tetragonality), which gives a reason for treating that as the tetragonality of the whole lattice with the ratio of its cells’ axes $c/a$ averaged over the whole crystal of the $\alpha$-phase.