mutable list operand in python

Question

Here is the 1st code.

lst=11*[[]]

print(lst)
word="Mia"
value=0
for i in word:
    value+=ord(i)
n=value%11 #Loaction number
lst[4]+=[word]
print(lst)

This output will be this [['Mia'], ['Mia'], ['Mia'], ['Mia'], ['Mia'], ['Mia'], ['Mia'], ['Mia'], ['Mia'], ['Mia'], ['Mia']]

The 2nd one.

lst=11*[[]]

print(lst)
word="Mia"
value=0
for i in word:
    value+=ord(i)
n=value%11 #Loaction number
lst[4]=lst[4]+[word]
print(lst)

This will produce this result. [[], [], [], [], ['Mia'], [], [], [], [], [], []]

What's the difference between lst[4]=lst[4]+[word] and lst[4]+=[word]?

I originally assume the abbreviation of lst[4]=lst[4]+[word] is lst[4]+=[word].

But they're apparently not.