why xy xy – what is the difference?

There is no difference. Regardless of what kind of syntactic sequence you use, the same byte code is generated.

>>> def f():
...   return 0, 1
...
>>> import dis
>>> dis.dis('[a,b] = f()')
  1           0 LOAD_NAME                0 (f)
              2 CALL_FUNCTION            0
              4 UNPACK_SEQUENCE          2
              6 STORE_NAME               1 (a)
              8 STORE_NAME               2 (b)
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE
>>> dis.dis('(a,b) = f()')
  1           0 LOAD_NAME                0 (f)
              2 CALL_FUNCTION            0
              4 UNPACK_SEQUENCE          2
              6 STORE_NAME               1 (a)
              8 STORE_NAME               2 (b)
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE
>>> dis.dis('a, b = f()')
  1           0 LOAD_NAME                0 (f)
              2 CALL_FUNCTION            0
              4 UNPACK_SEQUENCE          2
              6 STORE_NAME               1 (a)
              8 STORE_NAME               2 (b)
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE

In every case, you simply call f, then use UNPACK_SEQUENCE to produce the values to assign to a and b.

Even if you want to argue that byte code is an implementation detail of CPython, the definition of a chained assignment is not. Given

x = [a, b] = f()

the meaning is the same as

tmp = f()
x = tmp
[a, b] = tmp

x is assigned the result of f() (a tuple), not the “list” [a, b].

Finally, here is the grammar for an assignment statement:

assignment_stmt ::=  (target_list "=")+ (starred_expression | yield_expression)
target_list     ::=  target ("," target)* [","]
target          ::=  identifier
                     | "(" [target_list] ")"
                     | "[" [target_list] "]"
                     | attributeref
                     | subscription
                     | slicing
                     | "*" target

Arguably, the "[" [target_list] "]" could and should have been removed in Python 3. Such a breaking change would be difficult to implement now, given the stated preference to avoid any future changes to Python on the scale of the 2-to-3 transition.