It could just be that you have different curl patterns throughout your hair... many do. I have the same thing actually. Another reason I think this can happen, for myself at least, is that I have shorter layers in the front, but my back is all long length so it's weighed down more. Also I've seen people say that certain techniques give them less curl in the back like plopping.
Fine, low porosity, med/high density
BSL - going for mid back to waist
HG: currently just washing with a low poo that contains protein (Alba Naturals Coconut) and using Suave Naturals Coconut as both a RO and LI. No stylers right now, but this gives me pretty good curl, definition, low-ish frizz, and it feels and looks good. Also an occasional PT (liking the Palmer's Coconut Oil Formula which has cones) seems to be key for me.
Modified CG since February 2013