The idea behind an uneven split is that, in a lot of marriages, the partners do not act fully independently of each other. In marriages where one partner stops working to have or raise kids, they do so knowing the other partner will continue to work. Splitting according to the ratio before marriage would suit this (extremely common) type of relationship very poorly, because the partner who raised a child would have an interrupted career while the other wouldn't. It would be unfair and would discourage this (normal, often desirable) type of relationship