It's actually a different ranking, with different requirements. http://en.wikipedia.org/wiki/Woman_Grandmaster#Woman_Grandmaster_.28WGM.29
The November 2010 FIDE rating list records 254 women holding the WGM title alone and an additional 24 who are full GMs. I don't know whether Jennifer Shahade holds a full GM title (her Wikipedia article doesn't say she does); hence the distinction.
ETA: Her FIDE rating is 2307, which qualifies her as a FIDE Master, but is well short of the 2500 that's the norm for GMs.