Working on motor scooters has yet to transform me into a master mechanic. One of the things I will never understand is why you tighten a 3/8 hex head bolt with a 9/16 wrench. A 9/16 bolt requires a 1/2 wrench, and a 1/4 bolt requires a 7/16 wrench. Metric bolts are a similar situation, but at least you can do it without fractions.

Yes, I've noticed that hex head bolts are larger on one end and smaller on the other. They work better that way.

Another thing is thread count. Sometimes we refer to a 1/4 20 bolt, which means that it is a quarter inch in diameter, with 20 threads to the inch*. But more often we just say course thread or fine thread.

Any logical explanations will tend to confuse me further.
Howard
*You still have to use a 7/16 wrench if it has a a hex head. Some have square heads, and some have round heads for phillips or slotted screw drivers.