Behodar:

The patty is supposed to sit directly on the bun (to soak up the juice). Having lettuce there causes the juice to run onto your fingers.


Edit: Supposedly. Although I now note that a Big Mac has lettuce below the patty...



In-n-out burger, one of the awesome local burger chain in California does their burgs like the apple emoji too with the lettuce under the patty. Maybe it's a local thing?