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Numerical simulation giving different results in Python 3.2 vs 3.3

This may be a strange question, but here it is:

I have a numerical simulation. This is not a very long program, but somewhat long to explain what it does. I run the simulation a thousand times and calculating the average result and variance, and the variance is pretty small, on the order of 10 ^ (- 30).

However, I noticed that when I run the program in python 3.3, everything gets weird. See In Python 2.7 and Python 3.2. I always get the same answer every time. Same mean values, same tiny deviations.

But when I run it in Python 3.3, I get a different answer each time. That is, different averages and different (but still tiny) deviations. This is extremely strange, because the laws of probability say that this cannot happen if the variance is actually small. So I'm wondering what the hell is going on in interpreter 3.3, which has changed from 3.2, which makes my simulations go crazy?

Here are some things I thought of:

  • I may have a weird 32-bit / 64-bit mismatch in my versions of Python, but I haven't checked, and they all work on the 64-bit version.
  • I may have some errors in the float / int conversions, but this will be taken care of in Python 3.2, since they make a float with the return of the division if necessary, so the results of 3.2 and 3.3 should be the same.
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$ python3.4 test.py
{'a': 1, 'c': 3, 'b': 2}
$ python3.4 test.py
{'c': 3, 'b': 2, 'a': 1}
$ python3.4 test.py
{'b': 2, 'c': 3, 'a': 1}

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