Decimal rounding occuring when not expected to. Why?

Hi,

we have some simple C# code that accesses a data table containing weights for a portfolio. Each weight is expressed in decimal with 4 digits, with the restriction that they must add up to 100(don't need to be positive, although in this case and most practile applications they are). For example, a set of weights for a portfolio containing 5 assets is the following:

0.1469
0.1594
0.1525
0.3887
0.1525
=====
1.0000

The weights are input via an Infragistics table control, linked to a .NET data table, the code below goes over each row (portfolio asset), extracts the weight and computes a running total:


// dtPoints is System.Data.DataTable object

double dblSum = 0.0;
foreach(DataRow dRow in dtPoints.Rows)
{
  if ( dRow[PortfolioPointInfo.ColumnNames.Weight]  == DBNull.Value ) continue;
  double dblWeight = Convert.ToDouble( dRow[PortfolioPointInfo.ColumnNames.Weight] );
  dblSum = dblSum + dblWeight;
}

The first 3 iterations are ok, but on the 4-th one the running total gets rounded:

Iteration #1: Weight = 0.1469, Running Total = 0.1469
Iteration #2: Weight = 0.1594, Running Total = 0.3063
Iteration #3: Weight = 0.1525, Running Total = 0.4588,
Iteration #4: Weight = 0.3887, Running Total = 0.84749999999999992

Why the rounding? Cannot understand why it is happening. All numbers use 4 decimals, so I expect the total to have 4 decimals as well.

Any suggestion to help understand this will be much appreciated!

Best,
Stefano