This is because the pronunciation of "historical" starts with a vowel sound (ə), making it sound like "istorical." English grammar rules dictate that we use "an" before words that begin with a vowel sound.
Examples:
* An historical event: The American Revolution was an historical event.
* An historical novel: She was reading an historical novel about the Victorian era.
However, if you are referring to something that is literally historic, you would use "a historical," as in:
* A historical artifact: The museum displayed a historical artifact from ancient Egypt.
The key is to consider how the word is pronounced. If the "h" is silent and the word sounds like it starts with a vowel, use "an." If the "h" is pronounced and the word sounds like it starts with a consonant, use "a."