If there is a question mark in the duration box for project summary, it means that a task somewhere has not been given a duration. have a look through, you should find it and be able to update it so that the estimated duration for the project becomes confirmed.
Overall duration is dictated not just by the time between start and finish, but by calendars used. It will give the duration in terms of the protect calendar, so if you have another calendar for specific tasks this can make a difference.
Rounded days (as in your 26.33) are often a result of using edays- if you are using these in the programme, or have 24 hour working calendars (same thing effectively) this may be the cause.