There are some gatherer rulings about this:
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When the life totals are exchanged, each player gains or loses the amount of life necessary to equal the other player's previous life total. For example, if Player A has 5 life and Player B has 3 life before the exchange, Player A will lose 2 life and Player B will gain 2 life. Replacement effects may modify these gains and losses, and triggered abilities may trigger on them.
Quote:
If a player can't gain life, that player can't exchange life totals with a player with a higher life total. If a player can't lose life, that player can't exchange life totals with a player with a lower life total. In either of these cases, neither player's life total will change.
As to why it works this way, I can't tell you anything other than "because wotc says so".