Why is a BOOL a signed char?

The other day a user reported a bug to me about a toolbar item that was disabled when it should be been enabled. The validation code (simplified for your benefit) looked like:

- (BOOL) validateToolbarItem: (NSToolbarItem *) toolbarItem {

    NSArray* someArray = /* arrray from somewhere*/

    return [someArray count];
}

It took me a few minutes to realize that -count returns a 32-bit unsigned int, while BOOL is an 8-bit signed char. It just so happened that in this case someArray had 768 elements in it, which meant the lower 8-bits were all 0. When the int is cast to a BOOL upon returning, it resolves to NO, even though a human would expect the answer to be YES.

I've since changed my code to return [someArray count] > 0; however, now I'm curious why is BOOL really a signed char. Is that really "better" in some way then it being an int?