Lost in Time
Chinese characters have undergone considerable evolution in the past 3000 years. Some were improvements that enriched the characters, others corruptions that set in over time.
A major set of corruptions set in at around 200 B.C., when a king ordered a nationwide burning of manuscripts. People forgot how many of the characters were written, and they were arbitarily written by the scribes. Partially successful attempts were made during later period to rectify the characters.
Archeological data that came out in the 20th century has enabled us to know how many of the characters were written during ancient times.
Karlgren's 'Sound and Symbol in Chinese' has an interesting account of the evolution of the characters.