Fit width would have the effect of adding black on top and bottom if your deliverable aspect ratio is 1:77 since the aspect of your source is 1.89. I suggest checking your deliverable and seeing if it's set to center extract. If so don't use the fit width or height buttons and then look at the deliverable tab to see if the framing looks correct. If so, render a clip and check it against the source.
v4, coming soon, would work exactly as described above, which should obviate the need for fit buttons. Let us know if that helped.