The scroll bar in the Pinegrow preview is counted as part of the website and is taking up horizontal space that would, in reality, be usable.
For instance, I often rely on the “vw” unit to size text, and this is how it looks in Chrome (and how it should look):
And this is how it looks in Pinegrow:
In both examples, the text is physically the same size, but in Pinegrow, the scroll bar is “eating up” space on the right forcing the text to break into a new line.