For starters, Windows error codes are unsigned.

The prototypes maybe defined as unsigned ints, but thay are treated as signed values.

Vis: using this C program that returns its first argument as the return code:

#include <stdlib.h>

int main(int argc, _TCHAR* argv[])
{
    for( int i=0; i < argc; ++i ) {
        printf( "%d : %s\n", i, argv[ i ] );
    }
    return atoi( argv[ 1 ] );
}
[download]

And running this command line:

C:\test\test\test>for /l %i in (-5, 1, 5) do test.exe %i & echo !error
+level!

C:\test\test\test>test.exe -5   & echo !errorlevel!
0 : test.exe
1 : -5
-5

C:\test\test\test>test.exe -4   & echo !errorlevel!
0 : test.exe
1 : -4
-4

C:\test\test\test>test.exe -3   & echo !errorlevel!
0 : test.exe
1 : -3
-3

C:\test\test\test>test.exe -2   & echo !errorlevel!
0 : test.exe
1 : -2
-2

C:\test\test\test>test.exe -1   & echo !errorlevel!
0 : test.exe
1 : -1
-1

C:\test\test\test>test.exe 0   & echo !errorlevel!
0 : test.exe
1 : 0
0

C:\test\test\test>test.exe 1   & echo !errorlevel!
0 : test.exe
1 : 1
1

C:\test\test\test>test.exe 2   & echo !errorlevel!
0 : test.exe
1 : 2
2

C:\test\test\test>test.exe 3   & echo !errorlevel!
0 : test.exe
1 : 3
3

C:\test\test\test>test.exe 4   & echo !errorlevel!
0 : test.exe
1 : 4
4

C:\test\test\test>test.exe 5   & echo !errorlevel!
0 : test.exe
1 : 5
5
[download]

You can see that cmd.exe treats the return values as signed, otherwise the negative numbers input as strings would be returned as large positive values.

So, your "That's quite wrong", is quite wrong.