The difference Between Python's += and + Operators with Lists

I'm observing a discrepancy in Python's behavior when using the += and + operators with lists. Consider the following code snippets:

x = [1, 2]y = xx += [3]y += [4]print(x is y)  # Output is True

In this case, the output is True, indicating that x and y reference the same object in memory. However, when I slightly modify the code:

x = [1, 2]y = xx += [3]y = y + [4]print(x is y)  # Output is False

The output is False, suggesting that x and y now reference different objects. While += is often considered syntactic sugar for list concatenation, this behavior contradicts that understanding.

Could someone clarify the difference in behavior between += and + when used with lists? I'm interested in understanding the underlying mechanisms.