why aren't the frame rails of TT set evenly from the sides?

Jayco makes their own frames and talks about construction here on their blog.

If you think about it, the tongue goes back and connects with the frame rails, so in order for the point of the triangle (coupler) to be in the center, the frame rails have to be equidistant from either side.

I suspect that there is additional framing that you can't see under the coroplast bottom. Don't overthink it, just have the welder pout the bumper on.