If you go to UI Builder tab (not responsive) and then center the element within the parent, it’ll then be centered (unless it ends with an odd number of pixels to allocate, so then maybe shrink the element by 1px.). That should fix the 45px / 49px issue.
Note, in responsive the “Center” choice doesn’t fully center the element but instead has the element adjust based on the left and right margin as the page expands width. If you chose, say, left aligned it’d keep the element 49px from the left for as long as possible when the page is stretched or shrunk. So, there’s a difference between centering the element and setting the responsive setting for margins to centered.