I want to offer users a listbox where they can select % differences
In this case you should have some dimension (or goup of dimensions) in your data model so that you could perform such selection.
For example, the cusomer dimension is not sufficient, because the same customer can have different variances for different dates (I guess that's why you see other variances).
When you able to perform such selection manually, you could just use something like this as a listbox expression:
=aggr([your expression], ...dimensions to perform such selection separated by comma...)