[Tutor] Unexpected Result in Test Sequence

[kb1pkl at aim.com]

Hello Tutor list.
I'm running a test to find what the experimental average of a d20 is, 
and came across a strange bug in my code.
import random
list1 = []
def p():
    d = 0
    for number in range(1,1000):
        t = random.randrange(1,19)
        list1.append(t)
    for value in list1:
        d+=value
    print d/1000
    d = 0
for value in range(1,100):
    p()

It works, but I have a logic error somewhere. It runs, and the results 
have a pattern :
9
19
28
37
47
56
66
75
85
94
104
113
...
...
It just adds 10, and every second result, subtracts 1, till it gets to 
0, and then starts again with 9 in singles, and whatever in the 10's, 
etc.
What is causing this?