Why i can edit all the values in currency (exchange rate) except the last one

[9acca9]

Hi.
Why i CAN edit all the values in some currency, but not the last day?
I have "DATA Gathering" disable.

And well, i need to change the values in the currency. I change all the values. PERFECT... except the last one (not perfect :-)
Why? there is something i can do to change that day?

Thanks.