Every drawing program has a limited precision, that's for sure. No problem with that.
Inkscape let you decide what precision you want:
Menubar | Edit | Preferences | Input/Output | SVG Output | Numbers
- Numeric precision max. 16 digits
- min. exponent -10, i.e. numbers <1e-10 are considered as zero

For example, if you enter the size of a Rectangle, Circle, Ellipse, etc. in the "Tools controls bar" you can check the size in the XML-editor (Shift+Ctrl+X).

Three example cases:
a) Assuming you type in a rectangle width 40 mm, you will see width 40 in the XML-editor. Fine!

b) Although the "Tools control bar" displays only 3 digits after the decimal point you can enter more digits there.
If you enter 10.0005 you will get for example 10.0004997253418 in the XML-editor. Unexpected!

c) Create a guide, double click on it and enter 100, 200 for the origin.
What you will get in the XML-Editor: 100.0000002623867,200.0000005247733 which is unexpected. I never got 100,200 for the guide.

Well, yes, you can tell me that this is academic or this and that precision is enough for practical purposes.
However, if a program tells me that it has a certain precision but in reality it creates some arbitrary random numbers I would like to know the reason why it does so.
This looks to me that Inkscape claims to allow for a precision of 16 digits but in reality it handles maybe just only 8 digits. Is it maybe some internal LongInt, ShortInt, Extended, ... or whatever back and forth number conversions?
If Inkscape calculates internally with 8 digits, then an output of 16 digits is just nonsense.

Maybe somebody can explain to me?

Could it be some unit conversion? Which unit are you using in your file?

theozh, you're learning so much, so fast! Your questions are reaching to the point lately, where developers can probably give you better answers. That's speaking for myself, of course.

As I mostly observe, as I read the development mailing list (what part of it I actually read and can understand) I often see reference to "rounding errors". I don't know if your particular example here falls under the category of rounding errors. But what I gather from the reference to rounding errors, is that first, I think developers would like Inkscape to be absolutely precise. But second, that it sounds like it would be hard to do. And third, as primarily an illustration tool, Inkscape is pretty awesome, even with these very small errors. That's my impression. It could be right or wrong impression, but that's as far as I can understand.

I would suggest that you might want start reading and/or participating in developers' discussions. The wiki has been groomed as a developers' reference, and for the most part, all the user info has been transferred to the website. There is some developer info on the website, but it's mostly there to attract and be a starting point for new developers. So the wiki is one place you can start searching for answers (if you don't already). http://wiki.inkscape.org/wiki/index.php/Inkscape

Of course the development mailing list is there. Or if you don't want to subscribe, you can read the archives. Although I suspect it won't be long before you'll want to start replying to certain threads. This is a starting place for all the mailing list archives, although the Devel and User lists would probably be the most fruitful for you: https://sourceforge.net/p/inkscape/mailman/.

And from this page, you can find how to subscribe to mailing lists, as well as the address for the development IRC. https://inkscape.org/en/community/

So I would refer you to the developers, to answer this question

(Why most developers don't visit here, much less participate, is a mystery to me.)

I'm gonna take a wild stab and put this to rounding when display coordinates are converted to SVG coordinates. (The origin being different for each.)