Mad Dumb Men

I saw a car number plate with the letters MDM in the school car park today. Apart from having some numerical meaning for Romans, it was also part of the number plate of the Ford Granada which belonged to my parent's neighbours when I was growing up. A rather random and peculiar bit of information to carry around with you for all these years, by my reckoning at least thirty, but it's imprinted in my brain because I made out of those three letters the mnemonic "Mad Dumb Men". Why is it that I can remember something like that and yet can forget to take my daughter a snack for after Rainbows?