Thanks. So 2686 - 2327 = 359
The horizontal image is 260 pixels taller than the vertical image.
What isn't glaringly obvious in the documentation is that the PNG format uses a byte to signify "start a new row of pixels".
Yet even so, 359 - 260 still leaves 99 bytes unaccounted for. TBH, I don't know what those bytes are specifically used for. But unless you have some kind of transparent border not displaying that would explain them, I am sure they must be for some instructions that are needed to produce the image.
In any case, I wouldn't worry much about a few bytes here and there, that would be what I consider to be "over optimizing". IMHO, try both horizontal and vertical. If one is easier to work with / causes less problems, then use that image. Go after the "heavy weights" that could be slow-down bottlenecks first and worry about the "light weights" when they seem to be a problem.