Illuminance is what a surface actually receives, while candela describes how tightly a fixture focuses that output in one direction — the same total lumens can read very differently depending on beam spread. Because illuminance falls off with the square of the distance from a point source, small changes in mounting height change perceived brightness far faster than small changes in fixture output. The eye's own sensitivity also isn't flat across wavelengths — it peaks around 555nm, which is what the lux unit is weighted against.