If the size of an ounce were to fluctuate from market to market (as it often did before the standardization efforts of the 18th Century), the unit fails to yield utility. We would do as well to use "a handful" to measure and trade goods. Big handed people get a leg up.
By definition, a unit of measurement is a standardized constant.
So it's interesting to come across a unit of measure that does change relative to circumstances and maintains its usefulness. Currency is an obvious example of a fluctuating measurement system.
Here is an old Chinese system for measuring the size and space of dwellings:
"A chien is a unit of space with a constant relationship between height, width, and length--the space between the supporting pillars, the floor, the ceiling. In a large house, therefore, the chein is large and in a small house, small. The usual house in this part of China was three or five chien. Some of the smaller side houses might be two chien. The partitions from pillar to pillar, front to back, could be put in or taken out at will. A room could be from one to five chien. In poor families, a chien tended to be a room."
from a footnote in "A Daughter of Han: The Autobiography of a Chinese Working Woman" by Ida Pruitt as told by Ning Lao T'ai-t'ai
I will update this post when I recover information pertaining to a system of time measurement in which the time between sunrise and sunset is the same number of hours everyday, every season. Not only does the length of an hour change every day, but day hours are different from night hours.