are you on treatment? a drop by log of 2 is ok for the first month for treatment. however, if you're not on treatment, fluctuations can also occur, not sure about the range though.
The log of 100,000 to the base 10 is 5.
The log of 10,000,000,000 to the base 10 is 10.
Logarithms to the base 10 are generally used as a way of expressing viral load for HCV. 100,000 IU/ml is a low viral load, and within the normal range of what to be expected. Such a low viral load would give you a greater chance of success of treatment.
10 billion IU/ml is however an unlikely viral load. I suggest you check that number out. Viral loads may vary, but not to that extent.