Reward Points module- points not calculated correctly

Rewards points calculated in the step 2 of 3 of my checkout process is different as my rewards point side box, which is showing the correct points.

**here's an example from Step 2 of 3 - Payment Information:

Sub-Total:S$94.60
Registered Mail:S$2.20
Quantity Discount:-S$8.46
Total:S$88.34
Reward Points earned:86**

Correct Reward points earned should be: 88

My registered mail uses a table prefixes, and i notice it always minus 2 reward points from the actual points whenever registered mail option is chosen.

I understand that i need to change something if i'm using any table prefixes. Which is the file i have to change or add on?

**another example with the use of gift vouchers:

Sub-Total:S$94.60
Registered Mail:S$2.20
Quantity Discount:-S$8.46
Gift Certificates:-S$10.00
Total:S$78.34
Reward Points earned:66**

Correct Reward points earned should be: 78

the calculation is totally wrong here. could it be that my settings is wrong?

Appreciate if anyone could guide me on this. Thanks!