NXT motor power accuracy
From the page you linked to we can see that the NXC OnFwd method has the following parameters:

void OnFwd (byte outputs, char pwr)


The power or speed parameter is listed as a char type, and looking around elsewhere on the site we find the definition of the char to be:


  In NXC the char type is a signed 8-bit value. This type can store values from SCHAR_MIN to SCHAR_MAX.


This means it can hold an integer value between -128 and 127.

It cannot take any decimal values.

I assume that a char was used over an int as they only need to support values between -100 and 100 in this method.