Uneven layer width issue  

Uneven layer width issue

Hi guys,

Yesterday I printed a new phone holder to use in my car. Fitment was perfect but I noticed the two small walls are of uneven thickness.

So I thought it might had something to do with nozzle temp.
I made a temp calibrating tower first 1 shell thick then 2 shells thick. The single shell tower looks flawless:

I could hardly see any difference in temp changes from 215 to 175.

I then decided to do a 2 shell tower and here is where it gets interesting. Looking closely you can see half way up every 15mm the gloss turns more matte around 190 and lower temps:

Ghosting is almost completely absent:

The single shell wall showed much more ghosting, but I never intend to use 1 perimeter for walls. And the little protrusions on the corners are from the file itself. I could see them in Repetier Host I used for the temp change per every so many layers.

What these temp towers showed me that every layer is the same width and tells me that I calibrated the extruder stepper and filament diameter near perfect, right? So I don't understand why the model I printed to hold my phone is showing slight imperfections in thickness. I assume this has something to do with the infill an how much the layers overlap.

What do I need to change in Slic3r to get rid of this?
here are my Slic3r settings:

Infill is 20% and cubic/rectilinear