Unix is using an 8-bit value as exit/return code (see man wait). This value is typically interpreted as an unsigned value by the shell.

Update: but you could use Perl to display it properly :)  For example

$ perl -e 'exit -111'
$ perl -e "print unpack('c',pack 'C', $?),\$/"
-111
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(possible range -128 .. 127)