Outer edge tire wear is caused by too much positive camber or too much toe-in. With the latter, both tires usually suffer if this is the case, not just the one.
Did you get a printout of the alignment specs? If so, look at the camber spec on the RF very closely.
It's also possible to have suspension component wear and the alignment specs may show normal because the vehicle is stationary. Once moving things may start squirming around.
It's also possible for a component to become tweaked a bit due to a large pothole, curb strike, collision, etc. and be overlooked during an alignment if the person doing the alignment is really not on the ball.
Damage like this may not even be noticeable to the naked eye at all.
You might take a tape measure and do this. Measure from the back of the RF wheel rim (not the tire) to the front of the RR wheel rim. Do the same on the other side and compare the measurements. They should be very close. If the right side is less than the left it's possible there is suspension damage on the RF, likely a control arm tweaked, etc.
Hope that helps.