To you, it's arbitrary. Think about what you just said. You don't work with these things every day. Notice that "specifications" is plural, meaning that there are other criteria to meet besides the ashtray top. A can that's too small, for example, would be filling more frequently, meaning time lost from other work. A steel can would rust out in about six months. A plastic one would be a fire hazard. Most outdoor cans are too bulky and would obstruct the aisles.
What you're doing is shopping for trash cans without enough information to know what is practical on this property and what isn't.