This is just a result of how Python interprets addition of lists. From the docs
sum(iterable[, start])
Sums start and the items of an iterable from left to right and returns the total.
Since sum starts by adding the first element of the iterable to the start argument, you have:
[] + [1, 2] = [1, 2]
Then it continues adding items from the iterable:
[1, 2] + [3, 4] = [1, 2, 3, 4]
[1, 2, 3, 4] + [5, 6] = [1, 2, 3, 4, 5, 6]
