I wondered if it's just a coincidence but I happened to sit on the back seats when it happened.
Why the difference?
The only explanation coming across my mind is the TRAVELING of LIGHT!
Since the front seats are much closer to the blackboard, the time the light needed to travel from blackboard to my eyes, positioned in the front seats, is apparently less then it is for the back seats. I need more time to gather information on blackboard if I choose the back seats and that's why I took notes more slowly.