It seems parodoxical: your skin is all shiny yet your complexion is grey and lifeless. The explanation is a little technical, because it's to do with the reflection of light. Oily skin covered with a layer of sebum doesn’t reflect the light in the same way as other skin types.
To be precise, the skin sends all the rays of light back in the same direction - that of the angle at which the light arrived. It’s a real physics problem! Normal skin sends the light back in lots of different directions, and that's what makes it look radiant.
But with oily and combination skin, the focus is on the shiny areas, and, as the texture of the skin is irregular, less light is reflected, resulting in a dull, muddy complexion.