Hi Tioman
I don't have an explanation for you, but thought I'd let you have my ideas.
Census are often wrong, mistranscribed etc etc.
Have you double checked the names above and below Charlotte on the census copy? Perhaps Charlotte was visiting friends at the time of the census and she has been incorrectly put down as living alone, the wrong house number in the wrong place?
Her parents may not have been dead just because you can't find them on the census, although this thought is rather dependent on where you are getting the actual census info from. Different sources of the same Census can give very different information. Perhaps you could check another source?
Also her parents could well have been on the island but their names mis-spelt (some of my family names are hardly recognisable on various census and the names are not so unusual as Duquemin!) From my experience, census records are much more reliable later on.
Unfortunately this is only touching the surface of various reasons why you havent been able to find out what happened. Personally I would doubt that she was living on her own at 11yrs old so I would assume there is another explanation.
Perhaps it would be a good idea to concentrate on finding out when her parents did actually die, and then come back to this census result armed with this knowledge?
And no.....you are not being dim!! We all have strange things like this cropping up from time to time, but with some luck & time, you may find the answer to the riddle.
Sorry not to be of more help
Regards
winto