This is partially because converting strings into floating point numbers accurately is a surprisingly subtle thing to get right; the following links describe a bug in a library function that has been around for over 2 decades:

http://www.exploringbinary.com/gays-strtod-returns-zero-for-...

http://www.exploringbinary.com/glibc-strtod-incorrectly-conv...

http://www.exploringbinary.com/a-closer-look-at-the-java-2-2...

http://www.exploringbinary.com/java-hangs-when-converting-2-...

http://www.exploringbinary.com/php-hangs-on-numeric-value-2-...