strange slicing with [:]

[Skaperen]

as i understand it, slicing with [:] yields a new copy of the sequence, or if assigned to, replace the entire sequence (for a mutable sequence). but i have seen an odd effect when using [:] for both the source and target. the effect is that nothing happens. if i do

a=[1,2,3]
b=a
a=a[:]
b.append(4)
print(a)

i would not see the 4 append to b. but if i do

a=[1,2,3]
b=a
a[:]=a[:]
b.append(4)
print(a)

i do get the 4 as if a retained a reference to the same list.

Output:
lt2a/forums /home/forums 55> cat t1
a=[1,2,3]
b=a
b=a[:]
b.append(4)
print(a)
lt2a/forums /home/forums 56> cat t2
a=[1,2,3]
b=a
a[:]=a[:]
b.append(4)
print(a)
lt2a/forums /home/forums 57> python3 t1
[1, 2, 3]
lt2a/forums /home/forums 58> python3 t2
[1, 2, 3, 4]
lt2a/forums /home/forums 59>

what's going on with a[:]=a[:]?