This might be as simple as the fact that you're spacing your learning out less. In college you learned things over a span of 15 weeks, giving you many opportunities for spaced repetition, which is one of the most effective methods to make material stick. Now I'm assuming you learn material in much less time than a semester, so there's less repetition and therefore less long term retention.
https://en.wikipedia.org/wiki/Spaced_repetition