Why does Retin-A 0.1 percent make my face lighter in some areas and how do I prevent this?

Thanks for your question.  I would definitely see your board certified dermatologist and make sure you are not dealing with an underlying problem rather than not being sure if the retin-a is making your skin lighter.  The retinoids don't really have a lot to do with pigment, and people all over the world of different skin tones use them safely, so your condition sounds unusual.  If there is truly a darker or lighter shade to your skin where you are using the product, I'd recommend stopping it and getting a professional evaluation by a dermatologist.  Keep using a good quality broad-spectrum, mineral based sunscreen every day to make sure the condition is not worsened by the sun, even if you don't spend much time outside.  See an expert and good luck!