The extra time goes to the places shorter itineraries have to cut. Anuradhapura as well as Polonnaruwa, so you see both ancient capitals rather than choosing between them. Horton Plains and World's End, which needs a dawn start and most of a day. A full afternoon in Galle instead of ninety minutes inside the fort walls. None of these are detours — they are simply what a seven-day route has no room for.
Nine days also works well as two distinct halves. A common shape is five days of touring through the Cultural Triangle, Kandy and the tea country, followed by four days that stay in one place: an Ayurveda or wellness programme, a stretch of the south coast, or a rainforest lodge at Sinharaja. The touring half is long enough to cover the island, and the second half is long enough to be a rest rather than a pause.
If you are choosing between nine and ten days, the gap is smaller than between seven and nine — by this length you have already bought the second nights. The tenth day usually becomes a third night on the beach at the end.