ITopologicalOperator.ConstructUnion Problem

Hello all,

I have been using Contructunion method to merge multiple polygons from source to target feature class . Recently I noticed that the target features are not aligning with source . They are off by 0.19-0.9 ft . Below is my code . Can anybody give me an idea why this could be happening ?

qfeature = qFeaturecursor.NextFeature();
while (qfeature != null)
{
if (qfeature.Shape.GeometryType == esriGeometryType.esriGeometryPolygon)
{
pGeoCollection.AddGeometry(qfeature.ShapeCopy);
}

if ((pGeoCollection.GeometryCount > 0) && (bgeometryerror == false))
{
IPolygon pPolygon = null;
unionedPolygon.ConstructUnion(pGeoBag as IEnumGeometry);
pPolygon = (IPolygon)unionedPolygon;
}