As a teacher I give grades when I see fit. Sometimes during class I will ask someone to show a technique, or explain something.
I would hate to have my students train for the next grade instead of trying to understand Aikido. Much debate has been had over this topic, so to each his own
Each method has its advantages and disadvantages.
Understand simply this: it is the way a teacher shows his/her appreciation of your hard work. Be proud for a (very) short time and go back to practise!