The field specified only generates data if the Pipeline report has a row for Estimates then this can have a value for the Days Open field this is because Estimates can originate from Opportunities. If the row shows data for an Opportunity then the field is blank since an Opportunity cannot originate from another Opportunity.
Opportunity A = was open for 50 days and Estimate A was created from Opportunity A
Opportunity B = is open for 60 days
Report shows the following days open:
Opportunity A = 0 days
Opportunity B = 0 days
Estimate A = 50 days
Note: Currently filed as Enhancement 122292.