Here's why:

* Mixtures are formed by combining two or more substances without chemical reactions. This means the components retain their individual properties and can be separated by physical means.

* Fixed composition refers to the fact that a substance always contains the same elements in the same ratio. This is a characteristic of pure substances (like elements or compounds).

Examples:

* Mixture: Salt and pepper. You can have different ratios of salt and pepper, making the composition variable.

* Pure Substance: Water (H₂O). Water always has two hydrogen atoms and one oxygen atom, regardless of the source.

In short, mixtures are variable in composition, while pure substances have fixed compositions.