Why when changing the position of a robot, it doesn't behave the same way ?

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Why when I change the position of a robot, its behavior changes ?

Example:

A box that moves with SetLinearVel like this tutorial. Each time I run a simulation without changing the position, in 10 secondes the box travels exactly the same distance. But whenever I change its position I get different distances.

position (x y z) --> distance 

0  0  0 --> 0.135921353716680
0  2  0 --> 0.135921353736750
0 -2  0 --> 0.135921353736816
0 -30 0 --> 0.135921353736851

The difference of the behavior is bigger if we use a complexe controller (ANN) and more complexe robot(a four legged)

0  0   0 --> 0.220854219648855
0  1   0 --> 0.218853657846420
0 -1   0 --> 0.275672159550973
0  150 0 --> 0.709099068032837
0 -150 0 --> 0.709099068032837
0 -90  0 --> 0.444111580328392