beginner question fibonacci

[Joon]

 >>> # Fibonacci series:
... # the sum of two elements defines the next
... a, b = 0, 1
 >>> while b < 10:
...       print b
...       a, b = b, a+b
...
1
1
2
3
5
8



 >>> a, b = 0, 1
 >>> while b < 10:
        print b
        a = b
        b = a+b

        
1
2
4
8

Why a, b = b, a+b isn't a = b; b = a+b ?
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