Since you're using a pivot table the different types of expression totals are greyed out.
The distinct count totals might be different since a certain activity might show up for both Jan and Feb but since we only calculate distinct values the total will just show 1 instance for the activity.
You'll want to use aggr() in your expression to aggregate the counts over each dimension combination and then sum them up. Something along the lines of:
sum(aggr(count(distinct activity), type, month))