... Looking at various specifications, I've noticed that published track widths don't tell the whole story, and are only a guideline. It appears different specs are used, such as center of wheel, back of disc mounting surface, etc.
"Track width" does seem to be an ambiguous term, and the real measurement we would want (to avoid wheel offsets) would be hub face to hub face.
"Track width" is not ambiguous: it is the lateral distance between tire centres on the same axle. Hub face is also clear, but is not the same thing: it is the track width plus the wheel offset on each side.
Some amateurs and incompetent professionals may measure hub face width and report that as track width, causing confusion.
Wheel offset is not bad. There is an ideal wheel offset for each hub design, and to fit required components within the wheel volume essentially all modern vehicles are designed to work with moderate (15 mm to 50 mm) positive (hub face outboard of wheel centre plane) offset.
The same vehicle is sometimes available with wheels of different offsets, resulting in different track widths even with identical hub face spacing. Too much change in track width by this method results in poor suspension and steering geometry, and inappropriate bearing loads. This is especially important with steering axles (not the usual swap situation considered in this forum), and with independent suspension.
Due to the suspension, steering, and bearing issues, the important dimension really is the intended track width, but hub face width is what can be readily measured when there are no wheels mounted, and the offset required to result in that track width must be known to select the correct wheels.