because you are asking for it to be accurate to 16 significant digits. This is a result of the way floating point math works on computers. GOOGLE "Floating-point error mitigation"

The error is your assumtion that anything that is computed by AutoCAD (which is almost everything) has the value you think it does i.e. the distance between two points is 0.2 where it could be 0.200000000000001 or 0.199999999999998. To all intents and purposes this is 0.2 since (equal 0.199999999999998 0.2 1.0e-13) will return T. AutoCAD's smallest unit is the angstrom (1.0e-10) so the error is insignificant as it is a 1/1000000th of an angstrom. The largest IIRC is the parsec 3.0e16 metres where such an error will be significant, but this is only because of the limited number of computational registers when doing the math, and who apart from astronomers or astrophysicist works with parsecs.