I don't know what it is but people in my family seem to be very reluctant to finish off a box of cereals.
They will happily use them until near the bottom - then they will carefully pour a bowlful, but manage to leave a couple of spoonfuls in the box.
Anyone picking up one of these more or less empty boxes will shake it and then put it back, moving on to another one.
So, every so often, when I realise it has happened again, I finish off all of these more or less empty cereal packets - yesterday I had a really strange mixture of about 4 different types! Suddenly that part of the food cupboard looked very empty...
Of course there is another downside to doing this - those boxes have to be recycled too, and the recycle bin is ALWAYS full when I go to it, so I have to empty it and put in a new liner, etc.
Maybe I should just do what everyone else does and just leave those more or less empty boxes in the cupboard?