Here's why:

* Cell number changes: Cells increase in number through cell division (mitosis) and decrease in number through cell death (apoptosis). This is essential for growth, development, and repair.

* Cell differentiation: This is the process by which cells become specialized in structure and function. For example, a single fertilized egg cell can give rise to all the different types of cells in the human body, such as nerve cells, muscle cells, and blood cells. This process is driven by gene expression, where specific genes are turned on or off in different cells.

Therefore, cell number changes and cell differentiation are both crucial processes for life and are not mutually exclusive.