The infill can be related to your wall thicknesses not being a multiple of your nozzle size. So that it will venture in between two walls to fill in that tiny gap. The overall gap over the entire perimeter can then still be larger than the amount you specified. However it is recommended to produce solid walls for strength of course. For some models this zigzag infill works really well and can smoothen out the layer. It all depends on your printer and your exact settings, this is very specific. I recommend also adjusting the flow rate, typically 100 will work if you also accurately set filament diameter, and make sure your print speed is high enough so it doesn't keep the model too hot causing distortions. We are used to seeing planar surfaces so we will easily see imperfections in straight sections, what you can also do is improve the design to make it slightly curved. Printing on the side was a good call structurally, just keep the other strut there in place as well in case it will carry heavy loads, to prevent torsional unlayering. Just keep the 'support everywhere' option in place, it will be easy to remove.