Summarized Saved Search Formula Field Skews data

I have a perfectly working project task saved search whose purpose is to show me how many employees I have on each project grouped by the Revenue Stream (custom segmentation field) on the project. To enable the grouping I used formula:

case when {job.custentity_hw_revenuestream} = 'Core' then {custevent_hw_resource} else null end

case when {job.custentity_hw_revenuestream} = 'PPI' then {custevent_hw_resource} else null end

Where: {job.custentity_hw_revenuestream} = the Revenue Stream field.

{custevent_hw_resource} = employee.

For some reason before any grouping the employee numbers are correct for both Revenue Streams. But soon as I add groupings then the employee count for Revenue Stream = Core is wrong. They are higher. When Revenue Stream = PPI the employee counts remains the same and is correct. Does anyone know why adding summary groupings skews my data for only one column yet the formula format is the same?