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 ?