Each fluctuation term is a weight times one checked squared deviation. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Squared contribution row 1
Deviation -2 squares to 4. Weight 1 makes contribution 4.
w(x−μ)2=1⋅4=4
Squared contribution row 2
Deviation 0 squares to 0. Weight 2 makes contribution 0.
w(x−μ)2=2⋅0=0
Squared contribution row 3
Deviation 2 squares to 4. Weight 1 makes contribution 4.
w(x−μ)2=1⋅4=4
The middle state has weight but zero squared deviation
Weight alone is not a fluctuation contribution. The middle row has weight two, but its deviation is zero, so its contribution is zero.