Hi ,
If the data in column D is the input , why are you talking of the values in column C ? After all , what is being displayed in column C is because of the formula there.
If you convert the format of column D to General , you can see that the input data itself has many such changes ; D3:D19 shows times in increasing order , D20:D30 is another section of increasing times , D31:D47 is the third section , D48:D59 is the fourth section , and so on.
The formula in column C is bound to reflect this up and down behaviour of the data in column D.
Narayan