Why are there cold days in summer and rainy days on a parade?
Not all things are equal. Your interest, your motivation, your purpose, where you, sit in the room, what pen you're using during the test, Whether your significant other is mad at you or not today, whether you like the instructor ...
I don't think there is an answer to your question except to say, Things change no two things are quite exactly the same. Even when you do the same thing over and over again you get the same results but not quite. It's another day and nothing in that day is quite the same as you've experienced in the days before. There program on TV is different, it might be raining today, Ythe pillow fell on the floor for a change and the newspaper is on a different spot on the lawn today, and the grass is longer.
It's the same as every other day but not quite the same.
Alex
