Two kinds of line, and they behave differently
Catering and drinks scale with the guest list: they are a price per head, so their total is the price times the guests. Everything else — the venue, the photographer, the flowers, the music, the rings, the transport — is booked for a date rather than for a number of people, so it stays where it is however many guests come. The page separates the two, because that separation is the whole shape of a wedding budget.
On top of the goods comes the service charge, which venues and caterers normally charge on the food, the bar and often the room hire. This page applies it to catering, drinks and the venue rental. Then a contingency is applied to everything: that line is not a fee, it is the pad for the plus-one who was not on the list and the booking that costs more than it was quoted at.