Clothes Washer Waste

The CA Plumbing Code (section 804.1) tells us that the standpipe receptor shall be max 30" and min 18" above the trap. It also says that the trap height shall be min 6" and max 18" above the floor.
I have “plumbers” installing traps as high as 25".
What is the intent of the code on the max 18" to the weir of the trap?