Definition misbehaves?

The definition 'Parameter_Table' has a starange behavior in my code.
Is there an error in this code or a bug in C51?

In cases 0x20 and 0x21 this code returns quite different (wrong) values compared to cases 0x10 and 0x11 ( or 0x30 and 0x31 ) respectively.
Instead of Parameter_Table(PARAMETER_NUMBER-x) it returns something like 'Parameter_Table(-x)+PARAMETER_NUMBER';

#define	PARAMETER_SIZE	13
#define	PARAMETER_NUMBER	8

union parameter_type {
	struct {
		unsigned char	start_1;
		unsigned char	params_1[PARAMETER_SIZE];
		unsigned int	check_1;
	} parst;
	unsigned char	table[PARAMETER_NUMBER][PARAMETER_SIZE+3];
};

#define Parameter_Start		(0x1000)
#define Parameter_Table(n)	(Parameter_Start+n*(PARAMETER_SIZE+3))
#define Parameter_Check		(Parameter_Start+sizeof(union parameter_type))


unsigned int test_defines (unsigned char tmp) {
//
	unsigned int	tmpw;

	switch (tmp) {
	case 0:
		tmpw = Parameter_Start;
		break;
	case 0x10:
		tmpw = Parameter_Table(0);
		break;
	case 0x11:
		tmpw = Parameter_Table(1);
		break;
	case 0x20:
		tmpw = Parameter_Table(PARAMETER_NUMBER-8);
		break;
	case 0x21:
		tmpw = Parameter_Table(PARAMETER_NUMBER-7);
		break;
	case 0x27:
		tmpw = Parameter_Table(PARAMETER_NUMBER-1);
		break;
	case 0x30:
		tmp = PARAMETER_NUMBER-8;
		tmpw = Parameter_Table(tmp);
		break;
	case 0x31:
		tmp = PARAMETER_NUMBER-7;
		tmpw = Parameter_Table(tmp);
		break;
	}
	return tmpw;
}

    37:         case 0x10:
    38:                 tmpw = Parameter_Table(0);
C:0x0DD2    7C10     MOV      R4,#koe(0x10)
C:0x0DD4    7D00     MOV      R5,#B?SMEM(0x00)
    39:                 break;
C:0x0DD6    8024     SJMP     C:0DFC
    40:         case 0x11:
    41:                 tmpw = Parameter_Table(1);
C:0x0DD8    7410     MOV      A,#koe(0x10)
C:0x0DDA    FC       MOV      R4,A
C:0x0DDB    FD       MOV      R5,A
    42:                 break;
C:0x0DDC    801E     SJMP     C:0DFC
    43:         case 0x20:
    44:                 tmpw = Parameter_Table(PARAMETER_NUMBER-8);
C:0x0DDE    7C0F     MOV      R4,#0x0F
C:0x0DE0    7D88     MOV      R5,#TCON(0x88)
    45:                 break;
C:0x0DE2    8018     SJMP     C:0DFC
    46:         case 0x21:
    47:                 tmpw = Parameter_Table(PARAMETER_NUMBER-7);
C:0x0DE4    7C0F     MOV      R4,#0x0F
C:0x0DE6    7D98     MOV      R5,#SCON0(0x98)
    48:                 break;
C:0x0DE8    8012     SJMP     C:0DFC
    49:         case 0x27:
    50:                 tmpw = Parameter_Table(PARAMETER_NUMBER-1);
C:0x0DEA    7C0F     MOV      R4,#0x0F
C:0x0DEC    7D98     MOV      R5,#SCON0(0xF8)
    51:                 break;
C:0x0DEE    8012     SJMP     C:0DFC