What is up With the Last Pound!

Why is it the last pound or two is the hardest to lose? Not that I am so close to the end--I still have a long way to go. But, it seems every time I close to the ten pound line I am stuck. Not really complaining because I did great this week. Just wondering why I always get stuck before I go over the line. Last week I had gained 2 pounds but this week I lost 4 so that is a good thing. But I am at 170.7 pounds. Really? I couldn't lose that last .7 pounds? And now we are going away for the 4th and I'll be lucky to get over that hump. But, I am determined. I am not going to give in because that last hump will put me in the 160's, hooray!!!!