Continuity mistake: The letters A and B on the bottom row on the blackboard differ styles between shots, being wobblier in the second shot.
Continuity mistake: When Merlin searches for the model plane the knot holding it swaps from neat to crappy between shots. (00:56:02)
Continuity mistake: When Ector and Kay fight, the mallet behind turns 180ยบ. (00:49:30)
Continuity mistake: Before Kay cuts the mop's handle, the mop is dark brown. After he cuts it turns to vanilla coloured.
Continuity mistake: When Merlin arrives from the future, he talks about it to Arthur and puts the crown on his head. The sword, resting against the throne, appears and disappears randomly all the time.
Answer: Part of it is the cliche of the abusive or angry step-parent or adoptive patent. Although this is something that does occur, Sir Ector just thought more highly of his own son and would rather see his son become king. He probably regrets having to take care of him, but felt obligated as a knight of Uther Pendragon. In addition, he's more of a brute and fighter and doesn't like learning (or magic). So when Arthur displays an aptitude towards learning over fighting, Ector has less in common with the boy and treats him as a servant.
Bishop73