Because, as I said earlier, it measures how often that particular word comes up. Keyword density is also known as keyword weight. The higher the percentage of keywords in relationship to other text, the higher your page will rank—to point. Many search engines, Google included, have gotten wise to the fact that extremely high keyword densities are probably contrived. Here's an example of how keyword density it measured. Let's assume the keyword phrase is "cat food."
Cat food is our only business.
Since "is", "our," and other stop words are usually not counted, there are three "words" in the sentence: "puppy food," (which the search engine counts as one word, since that’s what it’s searching for), "only," and "business." "Cat food" composes 1/3 of the sentence, or 33%.
Realistically, keyword density is almost never this high, nor should it be or your copy will sound very contrived. The recommended density is 3-7%. This means that your keyword should repeat 3-7 times for every 100 words.
Sure, that may not sound hard, but believe me--having 10 keywords and trying to repeat each one 3-7 times per 100 words of text is practically impossible. Instead of trying to do that (and having copy that sounds really weird), pick two or three of your most important keywords and try to use them 3-7 times for every 100 words.
