Taxes

I'm pretty bad with keeping track of where my money disappears to, but am fairly punctual about paying bills. So, a few weeks ago I got told by the tax office that I have to pay back some money after some recalculations (all things that I was expecting) which I did almost immediately.
I just checked into my bank account and noticed that they've re-transferred more than the amount they asked me to pay back to my account claiming I overpaid them.
Huh?

I should do the right thing and find out what's going on, I guess?