I've also noticed that exact dimensions get changed behind my back, and it bothers me somewhat. Fortunately I can afford to ignore it for almost all the work I do. Although I love Inkscape, things like that make it feel a little bit broken, even if there's a good reason.
I assume the reason is that there's some rounding and conversion to other units going on behind the scenes. But mm is a native SVG unit, right? It seems like these numbers should be stored with the same units & precision as they were entered.
Or perhaps the problem is related to Paul's other concern -- that this figure is a sum of other figures, and so there are rounding errors in the calculation. I'm just guessing.
With recent talk about using Inkscape as a PCB design tool (which I'm very interested in), this may become much more important.
In regards to Paul's other concern: as an artist, I do find it useful that the dimensions given are the total size of the object (bounding box, I assume). For many tools (rectangle, etc), you can get the "naked" dimensions easily enough just by using the tool. But I can also see the use of having a way to get the "naked" dimensions of arbitrary shapes. I don't know of a way.
- Bryan