How It Works?
There is no single or universal optimal percentage keyword density. Each search query is the only search engine to compare (or normalization) on top of the other document files to determine their specific threshold. Like "credit card" in some of the keywords naturally there will be a two-word phrase, while others may be more dispersed. In addition, some with great visibility, strong and powerful connection using the data profiles highly trusted sites can easily escape smaller than the more trusted site can have more repeat.
As a general rule of thumb when it comes to the frequency of keywords ...
From a trusted content inside corpus (such as a person's internal site search, or select a database of well-known and credible content creators), the higher is generally better.
From a wide range of external content corpus (such as general Web search, many people have an incentive to try to game the system), less than is generally better.