Its a good idea to think of investing as an activity with a timeline in decades. If I buy a ten-year bond at 4% annual interest today, and a year later the same government is selling nine-year bonds at 5% interest, nobody will pay me the face value of my first bond. A year after that, maybe an eight-year bond is yielding 3%, and people will pay me more than face value for the first bond. I only know how much I made after inflation when ten years are up.

So the error there is that purchases are not actually independent. If say dot com stocks have been growing ten times as fast as the rest of the stock market, they can't do that forever (eventually they will become the whole stock market, then the whole economy). If a government keeps offering higher and higher real interest on bonds, eventually it will default or trigger high inflation. So the wise investor buys lots of different things, knowing that today's darling will be tomorrow's ugly sister. I recommend a good textbook.