It is a phenomena my best friend and I discuss often. We call it "Sherlock Brain," (derived from the clip at the bottom of this post):
Sherlock brain- the brain's tendency to discard the things it doesn't need in order to retain the things it does. This includes all kinds of info that, for some people, seem paramount but to others, extraneous. It varies according to your profession, your stage in life, your personality, etc.
For instance, I know lots and lots about women's fiction-- authors, new titles, backlist titles, upcoming release dates, themes and premises and lots of other things that other people do not know or care about. This is important to me because I am a women's fiction writer and because of the coverage we provide on She Reads. I need to know these things.
This means that there's not room in my brain for other things-- things like the political climate in our nation, headlines, the latest gossip at my kids' school, and yes, the solar system. (That will make more sense once you watch the clip.) My brain sloughs off anything not pertinent to my work or family. It is on a "need to know" basis. If I don't absolutely need to know it, it's gone.
This is also why I love lists-- because once I make a note of it, my brain doesn't have to hold onto whatever it is any longer, leaving more room for the things it does. Things like writing ideas, children's schedules, family needs, etc.
So the next time you feel addled, confused, overwhelmed, etc. don't beat yourself up about it. Just remember the Sherlock Brain concept. You're not being forgetful, you're choosing to remember the things that are most important to you. Sherlock is brilliant-- and I think what makes him most brilliant is that he knows what to let go of, what to hold onto, and most importantly, not to apologize for it.
(Here's the clip that brought about the terminology. The part you're looking for occurs at about minute 1:32. But watch the beginning because... well... it's Sherlock.)