Actually, it doesn't. When a word is used in metaphor or analogy it is used outside of its usual meaning. There is never a standard metaphorical usage in that there is only one metaphorical usage. Sometimes a metaphor is repeated often, but that does not constrain it as the only possible metaphorical use. The meaning of a metaphor MUST be determined in the context. In Ephe. 5:21-33 we have a metaphorical use of “head of” and “body of” inferring an element of husband wife relationship. Chapter six speaks of parenting (although Paul points to a failure of fathers) which we know holds an element of authority. Also chapt. Six speaks of the relationship between master and slave. Both children and slaves are under authority; a child to both parents, and slaves primarily to the husband, but in daily life the wife. The husband and wife relationship stands distinct from the other two by the metaphor of “head of” and “body of”. All three speak to the elements in a type of household of that era. Not all households had slaves, but when they did the responsibility of running the masters slaves fell to the wife who was to oikodespoteo them. 1 Tim. 5:14
Oiko - home
Despoteo – rule, manage (our word despot was transliterated from this word)
So what is the difference in the husband wife section of 21-33? Husband and wife are to become as one flesh. It is not one over or against the other. Many who have acknowledged the metaphor have still only paid heed to the “head of” tweeking it to mean “head over”. But the metaphor is not about the husband as "head" or “head over” and “body under”. There was a metaphor used occasionally in the OT of “head and tail”. That metaphor usually held the meaning of leader and follower; the head lead and the tail trailed obediently behind. That is not the metaphor here. Note that the husband is not told to be a “head” and the wife is not told to be a “body”.
So what does “head of and body of” have a meaning of in this place? It is defined in the context. The two elements leading up to the discussion of husband and wife are sacrificial love (vs. 2) and mutual honoring submission (specifically vs. 21 but encompassing 18-21). First the wife is mentioned that she is to extend the self instigated (verb form of upotassoMENOI) mutually honoring submission of verse 21 to her husband. The idea is that such respect (see vs. 33) does not stop at the door to the marriage. The wife is further admonished to mirror the trusting submission that all believers have toward our Lord (who first loved us and gave His life for us see vs. 2 again). Then the husband is told to sacrificially love their wife like Christ (vs. 2 again). Paul describes what Christ did in hopes of having a bride, sacrificing Himself and nourishing, caring for His potential bride. Then Paul says that a husband is to love (agape not eros) his wife and care for, nourish her as if she were his own body.
Thus the picture is that the wife is to honor, trustingly yield and receive from the husband as if he were “head of” her and the husband is to sacrificially love, nourish, and care for his wife as if she were “body of” him. The “head of” and “body of” metaphor is to tie them together to picture life’s union of “one flesh”. Husband and wife are to become as one flesh. Without the head the body dies, without the body the head dies. They are intimately dependent upon one another.
Had we been speaking of “head and tail”, that would not be the case, since any animal can get along without a tail. But Paul is painting a picture to teach us how to maintain real godly life in a marriage.