As hardwood is a natural product it will have natural colour variations and knots. This is inevitable when buying a natural product, and most people embrace these slight variations as it adds character and charm to the flooring.
There are different grades of hardwood flooring and a Prime Grade (the highest grade of wood flooring), tends to be more uniform in colour and appearance and has less knots. Once a Prime Grade hardwood floor has been installed, it does have significantly less variation, but is still a natural material so will carry slight changes in colour.