It's hard to tell from the picture but if it is uniform on each side then chances are it is from where the support material has been deposited during the printing process. As there is currently no way to specify print orientation this is an unavoidable problem. Only thing you can do is attempt to sand/polish the dull side to bring up its shine a bit.
There may still be support material on the dull side. The support material is the same as the solid except not cured. You can remove the support material by soaking the model in a warm and mild caustic soda solution as follows.
1) Use 1/2 a litre of water at 40-50 celcius.
2) Add 2-3g (1/2 teaspoon) of NaOH granules (caustic soda / lye / draino) to the water and stir thoroughly until all of the granules are disolved.
3) Drop the model in and leave for 5 minutes - if the water is too hot, the plastic will soften, but even just boiled water does the plastic no harm and can be useful to re-shape the model.
4) If there are small crevices or ridges, use a soft brush to help remove the support material.
5) Finally rinse under cold running water.
You may need to repeat steps 3 to 5 to get an all over shiney finish.