Diffusion changes mean-square spread without choosing a mean shift. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Diffusion widens mean-square spread
Diffusion coefficient 2 and time 3 give spread 12 square metres.
2⋅2⋅3=12
Spread widens without choosing the mean
The checked spread value is 12 square metres, while the mean remains a separate vector.