I think that this is pretty common situation. At least, it happened to me multiple times already.
DR is not a real-time metric. It takes time to spider all the links, do the math and repeat that to keep numbers updated. Also, their algorithm is not fixed and they might be changing it more or less when needed. Sometimes, you can acquire great link, that is deleted in a month or so, and it could be reflected in your DR the way you're describing.
I'd say it's not a big deal. Nothing is perfect and your domain is sitting around 50. Small differences doesn't matter - it's not Google formula, that it has direct effect on ranking.