The 318px width makes sense. Looking at the structure of the view in the dev rig it looks the difference stems from the dev rig not having a containing div that sets a 1px border. On my channel when I inspect the panel there is this:
<div class="tw-border-l tw-border-r tw-border-t">...</div>
But no equivalent div in the dev rig. Seems like that should probably be corrected to give the best experience in the dev rig. Either the div.view should be set to 318px or there should be a div with a 1px border to give the most accurate preview of what it’ll look like on live.