Pivot table - Shows 'Count of' instead of desired 'Sum of'

I have created a Pivot Table with: Column: "Who" (text), Row: "Task" (text), and Values: "Time Value" (number - derived from a formula, converting time h:m to decimal). The Value is calculating as 'Count of Time Value', (giving the number of entries), but what I need is the Sum, not the Count. In the Pivot Table Fields dialogue, i cannot see the expected option to choose Sum instead of Count. Any ideas why?

(In a previous version of this spreadsheet, the Pivot Table worked OK, giving me the Sum of).