Why do we say "Six of one and half a dozen of the other"?

Meaning:

The difference between two options is negligible

Background:

The earliest known written use of the expression is in the 1836 novel The Pirate and The Three Cutters by Frederick Marryat:

"I knows the women, but I never knows the children. It's just six of one and half-a-dozen of the other; ain't it, Bill?" (Chapter IV: The Leak)

The expression has its counterparts in other languages, such as the French c'est chou vert et vert chou - "it's cabbage green and green cabbage".

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