Here's why:
* The Tibetan alphabet has 30 basic consonants. However, these consonants can be modified by adding a "dot" (which is a diacritical mark) above them. This dot changes the pronunciation of the consonant, sometimes drastically.
* Tibetan has a complex system of consonant clusters. These clusters can be formed by combining two or more consonants, adding even more complexity.
Therefore, while there are 30 basic consonants, the number of possible consonant sounds is much greater, considering the dot modifications and consonant clusters.