Just found out how to do it.
When I click on a Crosstab node member and then click on the aggregate button, I keep selecting the "Total" option, which means the aggregate row adds up all the rows by default.
What I did to correct this was to:
1) Select crosstab node
2) Click on Aggregate button, select "Aggregate option"
3) For the row containing the percentages, leave the properties Aggregate Function and Rollup Aggregate Functions as Automatic
Report Studio would naturally know how to handle the summary of this column. Really simple but for a newbie like me, quite a revelation... hahaha