The habitat worth adding is rainforest. Sinharaja is the last significant stretch of primary lowland rainforest on the island and it sits off the usual route in the south-west, which is why shorter itineraries skip it. It needs a full day and a night nearby to be worth the detour, and it is a completely different landscape from anything else on the trip — endemic birds, leeches, and canopy rather than open dry-zone scrub.
The other addition is depth in wildlife. Twelve days is enough for three distinct parks — Wilpattu or Minneriya in the north, Udawalawe for elephants, and Yala for leopards — which is the point at which you stop hoping to see things and start reliably seeing them. Safaris are a numbers game and three parks across twelve days gives you far better odds than one park across seven.
Practically, twelve days is also the shortest itinerary where we would build in a deliberate rest day in the middle rather than only at the end. A day with nothing scheduled around Kandy or the tea country tends to be the one people remember, and on a shorter trip there is simply nowhere to put it.
12 Days / 11 Nights