If you're trying this by indirect adressing, high internal RAM (0x80-0xff) data will be returned. SFR's can only be accessed directly. I suggest to use a jump table, where R7 is an index.
First, you have to calculate jump entry.

MOV A,R7
ANL A,#7FH ;80h is index 0
MOV B,#3 ;multiply a*3
MUL AB
ADD A,#LOW RTAB
MOV DPL,A
ADDC A,#HIGH RTAB
MOV DPH,A
CLR A
JMP @A+DPTR
RTAB: MOV R7,080H
RET
MOV R7,081H
RET
MOV R7,082H
RET
...
...
MOV R7,0FFH
RET

Code size is approx. 400 bytes and routine is realy fast.