stage storage

I've created some drainage areas using feature lines in a parking lot for storm water storage. One at the top of the area
and a small square at the bottom to simulate a drain inlet. I have 9 of these areas. I then created 9 separate surfaces
using those features as breaklines in surface. for accuracy, I've set the contour interval at 0.1'. using the stage storage
on the analyze tab I created a report showing the amount of storage in each section. the problem is one of the areas shows
the some of the same contour elevations twice with different volumes and on goes from low to high and then back to low.
anyone know why this is happening and how to fix it>

Also, it seems you get a much smaller amount using the stage storage method versus creating a surface, a top surface and then creating a volume surface
even when using the 0.1' contour interval. doesn't make sense to me. I can see a little variation but not something as significant as I'm getting.