Continuity mistake: When the movie begins and Merlin prepares tea for Arthur, the amount and shape of the cookies on the plate changes between shots.
Continuity mistake: During tea, after Merlin commands the couch to come walking, the books on the table change positions between shots.
Continuity mistake: When Merlin enters his place and pours water into the pot, the bookshelf behind has a bottle on the upper right corner. A shot later the bottle has been replaced for a thick book.
Continuity mistake: When the movie begins a squirrel sits on a tree with dry and broken branches. In the following shot the branches look different, with small branches having suddenly appeared.
Continuity mistake: The first shot of Merlin's house shows a dark green pot on the top of the shelf behind. A shot later it's dark orange.
Continuity mistake: The door of Merlin's hut is on the left when Arthur makes to leave, but is on the right after Merlin's packing song.
Continuity mistake: When Kay tells his father that he is now "Sir Kay" at the banquet celebrations in the castle, there is an eaten chicken and bones in front of him on the table that disappear in the very next shot.
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