Here's why:

* Magma Composition: Magma is a complex mixture of molten rock, crystals, and dissolved gases. The specific composition of the magma determines the types of minerals that will crystallize from it.

* Cooling Rate: The rate at which magma cools plays a crucial role in the resulting igneous rock.

* Slow Cooling: Slow cooling allows for larger crystals to form as minerals have more time to grow. This results in igneous rocks with a coarse-grained texture, like granite.

* Fast Cooling: Fast cooling leads to smaller crystals or even a glassy texture, like obsidian.

* Fractional Crystallization: As magma cools, different minerals crystallize at different temperatures. These crystals can settle out of the magma, leaving behind a magma with a slightly different composition. This process, known as fractional crystallization, can lead to the formation of multiple igneous rocks from a single magma source.

* Assimilation: Magma can also assimilate surrounding rocks, incorporating their minerals and changing its own composition.

Example:

A single magma source could potentially produce:

* Granite: A coarse-grained, light-colored rock formed by slow cooling.

* Diorite: An intermediate-grained rock formed by moderate cooling.

* Gabbro: A dark-colored, coarse-grained rock formed by slow cooling of a more mafic magma.

* Rhyolite: A fine-grained, light-colored rock formed by rapid cooling.

So, even though a single magma body starts with a specific composition, the process of cooling, fractional crystallization, and assimilation can lead to a variety of different igneous rocks.