The main explanation for the differences is that a married couple would have more money to put into an IRA. A median of 0 for single households states that at least half of single households do not have any retirement assets inside IRAs (and defined contribution accounts). Thus, fewer than 50% of single households have any retirement assets.
Work Step by Step
If a dataset has a median of 0, then at least 50% of the data set must be 0. (By the definition of median, this is possible.) Thus, at most 50% of the data set would have retirement assets.