Continuity mistake: Harry buys the book "One Thousand Magical Herbs and Fungi", but later, the title changes to "One Hundred Magical Herbs and Fungi".
Continuity mistake: While being sorted into houses, Harry looks up at the Sorting Stool, and there are only three people left to be sorted. Professor McGonagall then calls out the names of four more kids.
Answer: Harry has already experienced a few magical incidents, as children from Muggle families usually do. For example, changing a teacher's wig hair to a blue color. If Dudley's birthday is on a school day, his parents probably don't want the risk of a problem at school. It's easier to make Harry stay with Mrs. Figg all day.