How to find the value of all possible combinations of 8x8 multiplier(approximate compressors) in Verilog?

8x8 multiplier-verilog code:

module multiplier(multiplicand, multiplier,product);
  input [7:0] multiplicand;
  input [7:0] multiplier;
  output [15:0] product;
  ...
  reg [15:0] product;
  initial begin
    ...
    product = f1+f2;

  end
endmodule

Verilog Test Bench:

module test_bench;

  reg [7:0] a;
  reg [7:0] b;
  wire [15:0] c;
  multiplier one(a,b,c);

  initial begin
      a=1;b=13;
      #21 $display(c); //#21 is for the delay produced from the muliplier block
      a=2;b=10; 
      #21 $display(c); //why not 20?Instead I get 13 again even if I use $monitor(c)
  end

endmodule

Output:

13
13   

Here why I am I getting 13 in the second $display? How to get 2x10 = 20 instead? I am trying to dynamically vary the inputs to the one multiplier object and find the result? How can I do this?

Full code-click here

My aim is to find the ratio of correct outputs : total combinations for a multiplier using approximate compressors

Image - I need to compute the final column of this image, i.e the total number of correct inputs for all the combinations possible (65025 for 8 bit multiplication). Is there any other way to do this? Thanks!