This is normal for many airlines. What happens is that they only allocate X number of seats at a certain price. After X number of seats are sold the price goes up to the next highest level. Apparently your five seats is over the X number of seats that are allocated at the lower price.
What you can do is make two reservations. Try entering the number of people one at a time -- i.e. try for 1, then 2, then 3, to determine at which point the price goes up. If you can book three people at $284 then do than and then book the other 2 at the $332. If you are traveling with children make sure to put at least one adult on each reservation though just in case.