ArcGIS 9.3 buffering problem

[Chelsea Wu]

Hi,

I have a point shapefile which was projected to NAD_1983_StatePlane_California_II_FIPS_0402_Feet.
I creates 37.24 ft buffer for each point and was expecting a circle with area close to 4356 sq ft.
But I got 4336.1039 sqft instead when I calculated using "Calculate Geometry..."
Was I doing anything wrong?

I don't remember I had similar problem with ArcGIS 9.1 or  9.2.
Is it a bug of ArcGIS 9.3?
Does anyone can help explaining this?

Thank you very much,

Chelsea