Similarity Equation of Hammouda

[misstiq]

Hi everyone.....
I'm researching phrase based document similarity now. I'm using Hammouda's empirical equation. More Specific, I'm using the equation that's without level of significant. But i found something strange.

If two documents which are exactly the same are compared, the result of the equation is zero. And if two documents which are really really different are compared, the result of the equation is also zero.

That makes me confuse......
Do I make mistake in understanding the equation? or is the equation wrong?

Please help me.............

Thanks before