Explanation of VB to SQL type conversion please

I recently came across an issue with type conversion that I would like explaining to me.

We had a SQL 2000  stored procedure that had a parameter declared as:
@event_id numeric(9,0)

We used the VB.NET SQLParameter class to send the data to the procedure:
Public Sub New( _
   ByVal parameterName As String, _
   ByVal dbType As SqlDbType, _
   ByVal size As Integer, _
   ByVal direction As ParameterDirection, _
   ByVal isNullable As Boolean, _
   ByVal precision As Byte, _
   ByVal scale As Byte, _
   ByVal sourceColumn As String, _
   ByVal sourceVersion As DataRowVersion, _
   ByVal value As Object _
)

like so:
param = New SqlParameter("@event_id",
                                             SqlDbType.Decimal,
                                             0,
                                             ParameterDirection.Input,
                                             False,
                                             9,
                                             0,
                                             "",
                                             DataRowVersion.Default,
                                             CSng(strEventID))

This was working fine until recently, when suddenly the value arriving at the stored procedure didn't match the value we were sending it. I.E. we sent a value of 10207933, but the value arriving at the spoc was 10207930.

We fixed it by using CDbl instead of CSng, but I want to know why, as a Single can hold values up to 3.402823e38, far more than the value being converted.

Can anyone explain this to me please?

Many Thanks,
Janine
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