If I had to hazard a guess, our table cutlery (i.e. the stuff people eat meals with) has somehow managed to get really bizarrely standardized. There are basically only two sizes of spoon in the normal set - a regular spoon, which has a fairly consistent size, and then a "soup spoon", which is a bit inconsistent (sometimes it's a larger size, sometimes it's round rather than teardrop-shaped, etc).
So most likely they're relying on the "incidental standardizing" of normal silverware, and making a guess that most people's regular eating spoons are bizarrely similar in size.
Since we're on HN, there's probably a really interesting philosophical rabbit hole one could go down, concerning the idea of things that aren't officially standardized (by some standard-setting body), but end up gravitating towards almost being so, due to industrial pressures and consumer expectations.
Yeah, there's sort of an "expanded set" like you describe, but there's definitely a thing where there's a very standardized subset of "normal fork, normal spoon, normal knife".
(Which is to say you're not wrong at all, I'm just stressing how standardized that subset is).
You can really see it with plastic utensils. If you get them from a restaurant, the spoons and so on are pretty consistent - not absolutely so, but it's interesting how often they'll be a certain type.
So most likely they're relying on the "incidental standardizing" of normal silverware, and making a guess that most people's regular eating spoons are bizarrely similar in size.
Since we're on HN, there's probably a really interesting philosophical rabbit hole one could go down, concerning the idea of things that aren't officially standardized (by some standard-setting body), but end up gravitating towards almost being so, due to industrial pressures and consumer expectations.