Another common thought for how the not-seeing-in-mirrors thing came about is that silver was seen as something that harmed or warded off evil, hence its use aga

Another common thought for how the not-seeing-in-mirrors thing came about is that silver was seen as something that harmed or warded off evil, hence its use against werewolf and such in many myths. Way-back-when, the reflective backing of mirrors, at least those owned by the rich or well-to-do, was often achieved with a thin coat of silver. Of course that doesn't explain other evil creatures having a reflection in the same mirrors, but as the various mythologies have different mixed sources we can't expect an awful lot of consistency.