If statement and "not" logic [duplicate]

This question already has an answer here:

• Order of syntax for using 'not' and 'in' keywords 5 answers

Im using Python 3.4, operating system is Windows. It looks simple, but confused me.

This is going to add all different numbers from list1 to list2:

list1=[1,1,2,2,3,4]
list2=[]
for x in list1:
    if x not in list2: 
        list2.append(x)
print (list2)

Output:

>>> 
[1, 2, 3, 4]
>>> 

But these codes also working too:

list1=[1,1,2,2,3,4]
list2=[]
for x in list1:
    if not x in list2: #Difference is here
        list2.append(x)
print (list2)   

Output:

>>> 
[1, 2, 3, 4]
>>> 

Isnt it very strange? I just cant understand how can it possible. if x not in.. make sense, but why if not x in.. is working ? not x means opposite of x ?