This is because chromatography is used for separating different compounds or dyes that travel at different speeds into its individual components. It is included in its purpose or function that it will move the components at different speeds. The dyes which cover more surface area will travel more slowly.
Similarly if two substances which have same speeds, they are going to travel together and they won't separate as chromatography separates substances having different rates or speeds for travelling.
Chromatography is used to separate at least two different substances based on differences between the physical or chemical characteristics of the substances. Depending on the physical or chemical medium in which the substance are passing through the substances with travel at different rates depending on their interaction with the medium. If two substances have similar chemical or physical properties they will behave the same as they travel through the medium and thus will not be separated.
Chromatography is a method of separating substances. In school it is often used to separate the light-capturing pigments in plants or to separate amino acids. It is also common to use paper chromatography in schools. We calculate how far the solvent used moves along the chromatogram and we measure the distance each substance has moved. If you divide the distance moved by a substance and divide that by the distance the solvent travelled you get a decimal fraction called an Rf value. This can then be consulted against a table of values for that solvent, which identifies the substance. If two substances move the same distance they will have the same Rf value; therefore, you cannot distinguish them. This might be resolved if you turn the chromatogram around by 90 degrees and use an alternative solvent. In a different solvent they might move at different rates. This would then enable you to distinguish them.