That's just how floating point math works in every language. Python intro: http://docs.python.org/tutorial/floatingpoint.html

Comprehensive paper going into depth about many other issues: http://docs.sun.com/source/806-3568/ncg_goldberg.html

If you don't understand the above, your missiles may kill you and your rockets may explode: http://www.ima.umn.edu/~arnold/455.f96/disasters.html

However if you don't care about performance but want precision, you can use e.g. the Decimal module (which is a little annoying with syntax help from the compiler) or perhaps various wrappers for arbitrary precision libraries that are around.