Flesch-Kincaid Reading Level and Calculators

So, here's something interesting: I am developing a program to assist students in reading, and as such I am looking into automated systems for determining readability/difficulty of any given text. One popular automated algorithm is the Flesch-Kincaid Grade Level, which tells you a supposed grade level of text based on word length and sentence length. Seems simple enough, right? In fact, the formula is here:


Why, then, does every Flesch-Kincaid calculator I use return different numbers... sometimes drastically different.

Microsoft Word: Flesch Reading Ease - 51.9, Flesch-Kincaid Grade Level - 8.8
Joe's Web Tools: Flesch Reading Ease - 59.6, Flesch-Kincaid Grade Level - 7.8
Standards-schmandards Readability Index Calculator: Flesch Reading Ease - 37, Flesch-Kincaid Grade Level - 11 (I find this highly unlikely)