Prorated charges for plan changes

Hello all,

I am very confused about the prorated charges on plan changes even after I chat with two reps. 

For the billing cycle 1/21-2/20, I was on $20 data plan. I changed it in the middle of the cycle to $30 data plan, and made it "BACK DATE EFFECTIVE".

Here is my question, if it was "BACK DATE EFFECTIVE", why am I still charged $10.67 for prorated plan? Where are the days for these charges? The $30 data plan should cover the whole 1/21-2/20 billing cycle as I understand.