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

Thanks for this suggestion -- this may well be the cause. Perhaps I'll start trying spaced repetition learning.

This is an expected result of a storage system that has finite capacity being tasked with storing a lifetime of useful information.

Research on this topic points to the notion that the brain is pretty actively trying to ignore things and toss out information that is no longer useful. It seems that the heuristic the brain uses to determine what should be kept and what should be discarded is related to how well some new information fits with information already in memory and how often an area of memory is revisited.

If you were tasked with fitting a lifetime of useful information on a 100 petabyte hard drive, how would you go about it? When you inevitably run out of free space, how would you continue to store new information?

http://brainworldmagazine.com/learning-memory-how-do-we-reme... http://neuroscience.uth.tmc.edu/s4/chapter07.html

I agree completely about learning math faster, or maybe easier. My thinking has been that now we have a much better scaffold to construct knowledge on. Calculus, linear algebra, proofs, thinking in arbitrary numbers of dimensions, etc. are all much more ingrained than they were in undergrad, and as a result you don't have to spend as much effort on the details.

I wish I could say I didn't forget things when I was was in university, but I still remember the frustration of coming back from summer vacation and having forgotten half of what we learned last semester.