What caused it? Since we cannot see code that you have not posted, my crystal ball is freaking out. It refuses to tell me anything. Darn it! This is the last time I will buy a crystal ball that was at a discount because it was refurbished.
Seriously, this is just an inadequacy of the models used. Better modeling techniques can probably prevent/reduce such inperfections. But small deviations from reality are a consequence of EVERY mathematical model ever made. NO model is perfect. That is why they are called MODELS. They are only approximations to reality.
If you want a perfect prediction of what happens in any physical system, you need to get down to the molecular level, and probably below that. You need to model electron orbitals. You may need to model how the quarks behave inside the protons and neutrons, etc. And for all of this, even if your quantum chromo-dynamics models are adequate, then you will probably still need a computer the size of the universe to predict the result.
The point is, use better models, making your predictions more accurate. But even then, you need to accept that a model is no more than a model, and that there will be deviations from reality in any model.
