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The Absorption Measure Distribution (AMD) in the X-ray outflows of Seyfert active galactic nuclei (AGN) describes the distribution of absorbing column density as a function of ionization parameter. Up to now, AMD has been measured only for seven objects with high-resolution X-ray data that contain absorption lines from ionized heavy elements. Even though the number of measured AMDs is not large, they display a universal broad shape containing a prominent dip, for which the absorbing column drops by around two orders of magnitude. In this paper, we tested a range of photoionization models against the overall shape of the AMD as observed in Seyferts. In particular, we demonstrate that the shape of the AMD depends both on the spectral energy distribution (SED) of radiation which enters the outflow, and the density of the warm absorber (WA). The model that best reproduces the observed shape of the AMD is one wherein the gas density of the WA is of the order of $10^{12}$ cm$^{-3}$, irradiated by an SED whose optical/UV luminosity is 100 times higher than the X-ray luminosity. When the cloud density is higher than $\sim 10^{11}$ cm$^{-3}$, free-free heating dominates the entire absorber, and only one instability zone occurs, which is in agreement with observations.