I would call it WEIRD math to define duration using dates at one WBS level and sum of values within another WBS level.

• What is the logic of this?
• Why not inverting the calculation rules?
• When you can have multiple calendars and the software calculations are in hours rolling-up days can yield wrong values.
• Because the calculations for days are based on averages calculation of duration in days can be misleading. Although never as good as showing durations in hours if you have the urge to show days better show remaining workdays considering every workday duration in hours can be different.
•   a single activity with a calendar where different days have different work hours.
•   as you add resource calendars duration in days can get very weird, even if for a single activity.
• Adding more activities as well as WBS phases makes it worse at the WBS levels.
• For simple calendars and resource loading it make sense to use days as an option to enter activity duration.

Santosh is spot on with his comment.  If you change your user preference settings to hours they should match.

It has to do with calendars. Where activities within a WBS group contain a mixture of calendars, the Proejct Default Calendar is used to calculate duration at the WBS Group level.

If you display your calendar column you'll see why.