Here's why:
* "Desert fox" is a general term: It can refer to any fox species that lives in a desert environment.
* The fennec fox is one example: It is the smallest fox species and is well-adapted to desert life.
* Other desert foxes exist: Examples include the Rüppell's fox, the kit fox, and the sand fox.
So, while the fennec fox is definitely a desert fox, it's not the only one.