You are close. The issue is this
Interval(@1-@2 ,'DD') as "closing_interval_days"
The interval will store all the details (days, hours, minutes, seconds etc..) between the 2 timestamps but the interval() function with 'DD' is just a formatting measure. So the pie chart believes they are different even if that is not obvious. When you format the day dimension to show more details as below, its apparent
To resolve this replace it with this which will round it to whole integer days and it should work the way you want.
floor( Interval(@1-@2 ,'DD')) as "closing_interval_days"