This happens because of the order that data is processed. In this case the data is coming back from the database, a table calculation is being carried out and at the last step the null values are being hidden. So the null values are actually being taken into account in the calculations even though you don't see the resulting bar. To solve this simply hide the nulls (right click on the bar and exclude). It will fix the calculation and those values will still be available on other sheets unless you specifically filter them in those other visualisations.
Hope this helps