The problem is you have the cells formatted to only 2 decimal places.

The average of the first column is NOT 123.54. It is actually 123.53833333
But you have the cell formatted to only show 2 decimal places, so it 'Appears' to be rounded UP to 123.54.
When in fact, that is only being DISPLAYED that way, but the actual complete value with 4+ decimals still exists in the cell.
So when you're doing the *52/12, you're doing that on 123.5383333..NOT 123.54


Hope that helps.