Why is array order changed?

Question

I am confused by following result in python. The order of the array is changed. How is this valid? I used Spyder IDE to run the code.

arr = {'1','2','3'}
print(arr)
arr.pop()
print(arr)
arr.remove('1')
print(arr)

>>
{'2', '3', '1'}
{'3', '1'}
{'3'}