The five day plan is for people who like to arrive somewhere and still have a free hour. It keeps the Taj Mahal and Agra Fort on day two, and then does something the shorter tours cannot: two nights in Jaipur, so the Pink City is a morning of palaces and an afternoon of your own.
That second Jaipur day is where couples and families usually spend the difference. Some visit a block-printing workshop, some browse the bazaars for textiles and silver, some simply have a long, slow meal. None of it is fixed, which is the point.
Delhi is kept light on day one for arrival and rest. If you want a full day there, look at the six day plan; if you want the Taj at sunrise, look at seven.