In python, why does a,b=x,y returns differently from a=x, b=y?

Question

For the following 2 functions:

def fn1(nums):
    A = B = 0
    for i in nums:
        A, B = max(A, B), A + i
    return max(A, B)

def fn2(nums):
    A = B = 0
    for i in nums:
        A = max(A, B)
        B = A + i
    return max(A, B)

if nums = [1,1,1,1,1], why does fn1 returns 3 and fn2 returns 5? how is

A, B = max(A, B), A + i

executed differently than:

A = max(A, B)
B = A + i