Why did French admirals rank differently from British admirals?

Why did the French Navy appear to have no permanent rank above vice admiral?

Higher French titles were generally functions assumed by vice admirals rather than permanent ranks; exceptional dignity or fleet-admiral titles were rare.

The French system distinguished the permanent rank of vice admiral from higher functions such as vice-amiral d'escadre and admiral. François Darlan uniquely held the rank of admiral of the fleet, while the older dignity of Admiral of France was exceptionally rare. Because British admirals held admiral as a substantive rank, they could claim seniority over French officers merely exercising a higher function.