Why does python set the display in the same order if the sets are unordered?

This is the first time I will review the python language from the Python wikibook .

For sets, the following is mentioned - We can also have a loop move over each of the items in a set. However, since sets are unordered, it is undefined which order the iteration will follow.

and the given code example

s = set("blerg")

for letter in s:
     print letter

Output

 r b e l g

When I run the program, I get the results in the same order, no matter how many times I run. If the sets are unordered and the iteration order is undefined, why does it return the set in the same order? and what is the basis of order?

[ PS: Sorry if I misunderstood something very basic. I'm new to python]