How come one of the hydroxides in the phosphate group are more acidic than the other?
Because the rendering is not really accurate.  It would have been better to render the two dissociation constants without trying to associate them with specific hydrogen atoms.

Given the neutral phosphate group with both hydroxyl hydrogens attached, either one has an equal probability of being removed as a proton, corresponding to the lower of the two $\mathrm{p}K_\mathrm{a}$ values.  The drawing simply picked one of the protons at random.  Then with that proton removed and the functional group now negatively charged, the second hydroxyl proton has become harder to remove and that is what the higher $\mathrm{p}K_\mathrm{a}$ represents.