It's not just you, and it's been that way for a couple of years now. On the first and last posts on any given page, a block of space is reserved, most likely for the potential insertion of ad space. The reserved space is fixed. If you shrink the width of your window pane down, you can actually get down to no text actually being displayed.

Options include avoiding accessing the forum on devices with a small screen and running with as small of a font as you can tolerate.

EDIT: And yes, updating a post does eliminate the reserved white space. As you said, go figure.