Absorption of hydrogen by palladium causes PdH to become superconducting below T_c ≈ 9 K. Due to the presence of one octapore and two tetrapores per each Pd atom, it is believed that T_c of PdH_{x > 1}, should increase further. Here, using ab initio calculation we show that (i) H placed in tetrapores of PdH_x induces a wide optical gap in the phonon density of states, which significantly reduces the electron-phonon coupling, and that (ii) the energetically preferable octapores filled by H enable the 9 K superconductivity only. This scenario may close a long-standing problem of the high-T_c palladium hydrides. Moreover, simulating the pore population by H and D, within ab initio molecular dynamics, we are able to explain the inverse isotope effect in the framework of the Bardeen-Cooper-Schrieffer theory.