Because when you first picked 27, it was 1 out of 100 choices. Then I tell you that you either got it right, or it's this other number. None of the others are correct, only 27 or 44.
So you think your 1/100 choice was better than the one I'm giving you now? On average, you'll be right 1% of the time if you don't switch. If you do switch, you'll be correct 99% of the time.
Another way to think of it is: you choose 27 or you choose ALL of the other 99 numbers knowing that I'll tell you that 98 of them are wrong and you'll be left with the correct one out of that batch. One of those clearly has better odds, no?
Roberto Bolaño