I think it's just your interpretation of what the frame should do in this case.
If you select Absolute it's simple, it will use the center of the canvas and draw the frame around the perimeter.
Without Absolute selected I calculate the center point based on the extents of the Sketch, and then the frame is calculated using the 0,0 so in this case the center is X42 Y106 so the frame would be from 0,0 to 84,212.
The reason it's so large in the Y axis is you placed the text half way up the canvas.
I could take extents of the Sketch and use those for the corners, and not use 0,0 at all. If others agree that is a preferred method I will change the logic.