Back to the issue of hook sizing. I emailed Tulip Co., Ltd, the makers of Tulip crochet hooks, in Japan. These are, by far, my favorite hooks.
They look nice, they feel nice, they're ergonomic so my hand doesn't cramp too much-I love them. I "hinted" to my husband that a set would make a great birthday present, and he didn't disappoint. Thanks, hubby!
Well, not only does Tulip make great crochet hooks, they also have wonderful customer service. They got back to me in less than a day (time difference?), with a photo that explains a lot. At least to start with.
It seems the difference between hooks of the same size but from different manufacturers has to do with the change in diameter along the throat of the hook.
For reference, this is the illustration from the Crochet Guild of America's page.
It seems, from the photo from Tulip, that what's the same in hooks of the same size is the diameter where the throat meets the neck. What differs between hooks of the same size can be the diameter of the throat at a specific distance from the point where the throat meets the neck. So, in the image:
Both the Tulip and Clover are 2.2 mm hooks. But close to the pad, the Tulip's diameter is 3.3 mm, while the Clover's is 2.6 mm. (Another question: Do all crochet hooks have throats of the same length?)
I guess this means that as the loop of yarn slides up and down the throat while you crochet, it expands more or less, depending on which hook you use. Clearly, more on the Tulip than on the Clover. (Not to dis Clover hooks. I have several, and they're also very nice).
The next step is to try to understand how this effects the appearance of your stitching. I sense an experiment coming on. Stay tuned.