I am trying to solve FIBQ

using the editorial editorialFIBQ. Can anybody explain me why we are calculating the

L.sfibm1 = Fibonacci(A[i] - 1)

L.sfibp1 = Fibonacci(A[i] + 1)

If the input array is 5 ,7,8,10,12.

then fib(5), fib(7) , fib(8) , fib(10),fib(12).

then for intermediate nodes fib(5+7), fib(8,10),fib(5,7,8,10), fib(5,7,8,10,12).

Now merging logic for 5 and 7 can be simply fib(5+7). then we are using such complex logic as explain in editorial.

Please explain.