Clear out your browser every time you search as that affects prices shown. As others have stated each plane has a bunch of fare buckets which is why the guy next to you paid a different fare.

For what you are trying to do it might be good to contact a travel agent who can put together the fare differently as online will price out separate one way fares for multi segment. Also if you do a package with airline and hotel combo you can often save money over the piecemeal pricing. Tends to be worth travel agent fee IMO