who's your internet provider?
was the speed test run with the same server location?
different factors could have affected it. simplest explanation would be if bandwidth was being used by other connections off of your network.
if you tested both times from your computer, and you were the only one using the bandwidth and it was from the same server location... then its just your ISP being gay (sadly it could get worse, with the current battle for net neutrality that the US is having)
4-5 years ago i remember i worked at a call center, and in my department i worked with outsourced comcast calls... man that was a bitch
lot of cable providers, will have slower speeds at peak load times. because your connection is likely shared/split on a node.