When purchasing a solid hardwood floor it is important to remember that wood is a product of nature. As such every product is unique and the natural characteristics of the wood cannot be controlled. Knowing the distinct characteristics of the species you are purchasing can help mitigate any surprises and ensure you attain the aesthetic you want for your home.
The color variation in a hardwood floor is caused by two things. The natural color of the wood and way the wood absorbs the stain that is put on. Many manufacturers provide various grades (select & better, character) to provide a choice between more and less color variation. The main difference between these grades is the quality of the raw material used. A select & better product will use AB grade lumber which is clear of knots and other defects resulting in a cleaner product with less variation. A character grade would be made of CD Grade or Com 1-2. These will produce a high variation effect and allow you to see more of the natural wood character.
Some species are specifically sought out for their natural high variation. For example hickory and acacia both have very distinct grain patterns that produce beautiful high contrast flooring with a natural finish.
On the flip side species such as white oak and maple have less distinct grain patterns and more even color tones. However every wood species has some degree of color variation.