Other mistake: While Dr. Harvey is looking at his multiple reflections in the mirror over the sink, he sees the Crypt Keeper reflection. It then cuts to a close-up facing the real Dr. Harvey as he freaks out, and it's a flipped shot—note his shirt and sweater are backwards (this is not a mirror's reflection)—right before he gets his foot jammed in the bucket. (00:31:05)
Continuity mistake: After Amelia grants Casper's wish to be alive (albeit for only a short time), when it cuts to Kat sitting alone at the party, note the distinctive worn chairs beside Kat. In the following shots, those chairs not only move their positions, they also switch places with each other more than once. (01:27:50)
Continuity mistake: When James and Kat first enter Whipstaff Manor, they drop their stuff on the floor. The large suitcase falls over on its side, but it cuts to the overhead wide shot and the suitcase is standing upright (the ghosts didn't do it). (00:19:45)
Continuity mistake: After Kat finds a bedroom for herself, the long sheer white curtain hanging near the framed photo of Amelia repeatedly changes between pulled open or closed; particularly noticeable when Kat dumps her clothes from the black trash bag and tosses a ball of socks into Casper's mouth. (00:25:15)
Continuity mistake: Carrigan decides to make Dibs a ghost, so she goes after him with a double-headed axe. Then, when Carrigan slips on the muddy goo, her black pointy toe high heels turn into beige flat rubber-soled shoes, seen in her close-up as her foot lifts up. (01:11:45)
Answer: The Ghostly Trio were, at the time, playfully teasing James, so Fatso wearing something else instead of what Amelia was seen wearing in the photo, was just a way for them to have some fun at James' expense. At the end of the movie however, they did keep their promise and let James have one final chance to speak to his wife.
Why that specific outfit, though?
Again, to playfully tease James.