When we started adding the Gunning Fog index to the Word Frequency Analyser, we soon found that the same text could be given a very different score by two sites, though they were using the same formula.
One problem is with the number of three- syllable words. Should the same word be counted twice? Some places counted them once and some counted the number of occurences. For the moment, we have set the program to count them once, which seemed reasonable as the difficulty of a word will decline with repetition.