Why is the French naval ensign (shown here) not identical to the French national flag?

The current French ensign is not, as the casual observer might think, identical to the flag of France. Though both are blue, white and red, the French civil ensign -- also known as the French national flag -- has those colours in bands of equal width.

But this equal-width arrangement would not work well on a ship at sea: the fly of an ensign appears to shorten when moving in the wind. To counter this effect, in the 19th century, the width of each band was adjusted in the following proportions: blue 30, white 33, and red 37. The intention is to create a flag which, when seen moving at some distance, will appear to have columns of equal width; in addition, the slightly wider red column is intended to improve the flag's visibility at sea.