% incorrectly updating when slicing - ALLEXCEPT DAX

[Anonymous]

Hi,

I've created a measure to show me the percentage of responses to a particular question on a survey, over a number of years...

Measure = SUM('FYE Data (2)'[Response]) / CALCULATE(sum('FYE Data (2)'[Response]), ALLEXCEPT('FYE Data (2)', 'FYE Data (2)'[Year]))

Example of the table below...

When I drop the measure into s stacked bar chart and update to the corresponding year eg. 2020, the measure displays the correct percentages. However when I slice on additional years, eg. 2019 and 2020, the % displayed alters slightly, albeit by a few %. Any ideas what I'm doing incorrectly with the measure?