へうげ read as "hyouge"

[Qaantar]

A friend recently brought my attention to a manga series called へうげもの, which is pronounced as "hyouge mono" (the manga book has those letters in English on the cover).

The story takes place in the Sengoku period, and I am not familiar with most of the grammar and peculiarities of that era. Is the reading of へう as ひょう a function of older rules of Japanese, or is it something else?

[Ongakuka]

Spoiler:

http://en.wikipedia.org/wiki/Historical_kana_usage

[Qaantar]

Excellent, thanks!