Look fine on my PC, too. Incidently, the primary reason for this kinda thing is different Gamma factors on different systems. PCs run mostly about 1.7 to 2.2 (they are supposed to be 2.2, the NTSC standard), Macs run rather lower, originally they were supposed to use Gamma 1.0 I think (same as paper), typically 1.4 to 1.8 nowdays, though. I'd guess Unix workstations run closer to Mac than PC settings. The Gamma factor is the brightness curve vs voltage, the 2.2 gamma is supposed to match the response of the human eye, so both lights and darks have equal spacing.