The reason that you get better gas mileage on 93 is because the oxygen sensor closed loop feedback along with the knock control on newer cars allows the ecu to run more timing and less fuel. With the 87 not as much timing can be run and if it senses any knock noise it adds fuel as well.

The difference in 93 and 87 is the auto ignition or detonation point. The higher the octane the more pressure and temperature it takes to auto ignite or detonate the fuel.

Dan