The Atlantic tracks are set up differently than land routes due to the different methods of ATC surveillance. Over land, ATC has radar pictures of airplanes, and controllers can see what's going on. Should the need arise, they can issue commands to aircraft and resolve situations very quickly. Ove the Atlantic, however, the lack of radar requires a more orderly navigational routing system. ATC only knows the position of aircraft based on pilot position reports made every 5 degrees. Due to the radio system used for long range communication (HF as opposed to the VHF used over land), it can take up to 20 minutes from the time a pilot makes a position report, it is received by ATC, ATC issues a command, and the pilot receives the instruction. Needless to say, if two aircraft are converging, 20 minutes is not quick enough. Therefore, the aircraft need to be in neat, orderly tracks rather than flying however they want to. The tracks can, as Boeing727 said, be changed drastically from day to day. The changes are especially drastic if there is severe weather over the ocean that must be circumnavigated.