All (conversion) rates in my Piwik installation were integer valued (1.0%, 2.0%, ) instead of (1.2%, 2.6%, etc.). The reason seems to be the localization of PHP. My Server is located in Germany and float numbers are represented by a decimal comma:

#!php
$floatNumber = 10 / 4;
print_r($floatNumber);
// output: 2,5

However, MySql doesn't understand the comma based floats and just seems to cut of all decimal places resulting in the integer valued rates. I've solved it by adding the following code line somewhere in the code

#!php
setlocale(LC_NUMERIC, 'C');

But probably there is a solution in not such a hacky way.