Imagine you toss a coin. How much of the time would you expect it to show heads? Tails? About 50:50, right? How many times would you expect the coin to land on its side? Almost never?
Now imagine the situation with pills. Actually, you pop them out of the packet and it is surprising how often they land on their side. Especially when they're on a surface that isn't flat. They then proceed to roll away, sometimes right under the cushion or even under the sofa. Great fun!