The wavelength resolution for a particular BCS channel depends on the rocking curve of the crystal for that channel, on the position resolution of the detector, and on the angle that the photons enter the detector in that channel. The latter point is more relevant to the higher energy photons of channels 1 and 2 - these photons penetrate further into the detector volume and if their path is not perpendicular (to the window) they will produce an electron cloud that spans a longer length of the wedge-and-wedge cathode, and hence a broader positional distribution.