Occasionally, when backing vocalists, I hear them say 'I sing in Bb', or I usually sing in my favourite key - G'. They sometimes proved themselves wrong, when I had to change key to one which fitted their range better - didn't tell them -but where did this concept come from, and is there actually any sense in a statement such as that? I don't mean they say they sing a particular song in a particular key - good vocalists do this - but they state that they sing everything in x key.
That's absolute non-sense if it's meant as a general (i.e., song-independent) statement. Of course every singer has a certain range, and also a comfort zone. But the range of a song, i.e., its lowest and highest note, is not determined by the key, but by the way the melody is written. Of course, chances are that in a certain key the lowest note is the root or the fifth, and that the highest note is the root, but there are too many exceptions to use this as a guideline.
The notion of a favorite key for someone's voice is not as far-fetched as you might think. Professional singers intimately know their voices and their "money notes", the exact spots where they can achieve the ideal sound. These spots can be as delicately placed as a semitone, so it does make sense that someone likes singing in E as opposed to E flat, since their high e sounds a lot better than their high e flat.