While not like a man's I have a low voice for a woman. In my case it is based on my hearing impairment. I do not hear the entire spectrum of frequencies, rather those that mostly include the consonants - which tend to be lower. Hence, I developed a speaking voice in the lower frequencies too. It is also quite 'husky' which I have been told is quite appealing, haha. Think Lauren Bacall.

There are other reasons that come to mind like a goiter or active hypothyroid (both are related to glands in the neck). Whichever it may be there are medical reasons for having a lower pitch.

Many woman have soprano voices while some are lower at a contralto. I myself am even lower pitched at an alto. Having an even lower voice is not so common.