Hi @MSC,
Pivot tables are always going to sort by the lowest granularity, which for you is ‘Months’ in this case so unfortunately, you are unable to sort them in this way.
I’ll mark this as a feature request as different ways of grouping would be a nice feature to be added.
One work around you could utilize in this situation though; you could make two separate tables, one that focuses on the Volume (grouped by month) and the other on % (grouped by month). Then, you can hide the rows from the second table so that you can move it in line with the first. Setting this up in free form will make it easier for them to align with each other and look as one.