Other mistake: When Ben Stiller and Robert De Niro went to the store there are head rests on the back seats, but on the way back from Owen Wilson's house they've gone.
Other mistake: Near the end of the movie, Greg says he's in row 8 on the plane. But when he's on the plane, we see there are only about 7 or 8 rows behind him. If the plane had only 15, 16 rows, the plane wouldn't have that middle section of seats.
Other mistake: When Greg asks the kid behind the counter if they have the Nicorette patches, he says no, and hands him a pack of the Nicorette gum. However, the package to the left of the gum, and possibly to the right, are patches.
Other mistake: When Ben Stiller borrows clothes from Robert DeNiro, he was wearing a beige turtle neck sweater. In the breakfast scene when Owen Wilson is introduced, Teri Polo wears the same sweater for most of the rest of the movie.
Answer: Greg was probably surprised at just how "richy" they really are. Because lacrosse camp seems like a really rich, preppy thing to do. But mainly, I think he was suprised because he did not know that his girlfriend's ex and his girlfriend's sister's fiance knew each other. He didn't realize how connected they all were.