Languages and Linguistics

The first alphabets were proposed by missionaries, who typically were familiar with Latin and Italian, often German, French or Portugese as well. The phonemes used in Chinese do not correspond exactly to those used in Western languages, so some of the mapping between Chinese sounds and Roman letters had to be a bit arbitrary. The current system is sort of a hybrid of other systems proposed historically by Westerners and Chinese scholars, revised by native speakers in multiple iterations.

For more, see the Wikipedia entry for Pinyin.