Desktop preview changes sizes even though the artboards are the same size?

Hi,

Designing a website on mobile. I have 3 artboards, all 375 width. Desktop preview works as I would expect on artboards 1 and 2, but when going to artboard 3 it appears crunched-down in size with about 20 pixels of black space on either side of the art. I cannot figure out what is happening. The 3rd artboard is definitely the same 375 with as the others.

Chris