Some faces appear darker BUT normals are okay I have noticed that sometimes when I model some objects, some face later on appear to be a bit darker than others. Especially after texturing.
I have checked the normals and they are pointing in the right direction. I have tried recalculating normals to see whether it fixes anything but no luck.

I also tried rotating the objects to see whether it is an issue with lighting. Sadly it isn't.

Also, this isn't caused by a normal or bump texture because the problem appears even before I start adding those. 
This is the third model I am experiencing this on.

I will appreciate even just an explanation to understand why this happens.

Thanks in advance.
when this happens to me i just select it and apply "faces>shade flat". if someone has a better answer please share.