Comment 5 for bug 772057

> Why does the line spacing and word spacing work (…)?

1) line spacing is a relative size (in %, or "times font size") and thus not affected
2) editing the word spacing value shows the same effect as editing letter spacing (1px -> 0.8px)

> If you think it's the initial program's fault

It's no fault per se - the 'viewBox' attribute is valid SVG code (which is partially supported in Inkscape). After further tests, Inkscape's GUI does indeed have some inconsistencies with regard to units if the attribute is present compared to how the scaling is compensated in other widgets, e.g.
- Fill & Stroke > Stroke style > Width
- Text tool controls bar -> Font size
do compensate the scaling and read/store the scaled value to the SVG source (enter 1px in the GUI, write 0.8px in the SVG source; read 0.8px from the source -> represent as 1px in the GUI). The code for the extended formatting options on the text tool controls bar is rather recent (new in 0.48), and apparently doesn't use the same concept how to handle parent transforms or scaling via viewBox attribute. [1]

With regard to usage of the 'viewBox' attribute in Inkscape: The problem how to best handle the need for absolute, real units based on the SVG file format (which doesn't really define them as absolute values) is a recurring discussion on the developers' mailing list - the viewBox method (use SVG user units (px) for all elements in the drawing and scale the drawing contents via viewBox attribute) is known and (AFAIU) recommended by the standard, but not (yet?) used by Inkscape for setting the drawing units/scale of its own drawings.

[1] see next comment