Neil, thanks, an explanation that I found interesting--"Oh, that's why it does that!"
For what I do, other than looking odd at high resolution, the offset creates no difficulty. In AM mode, it really does not matter and in sideband I tune by ear. (What's the likelihood that my 7 parts in 10 million is more accurate than the frequency control of their machine?)
If you are willing to continue to be helpful, what is an "FFT bin?" The vertical lines in the graphical part of the IF display? Something else?
p.s., I'm a "math, science, and engineering guy," not a "maths, science, and engineering guy!"