DataTable Fill

I am running into a little problem in filling my datatable from an
access DB.

driverVehicleComb is a DataTable

driverVehicleComb .Clear();
oleDbDataAdapter1.SelectCommand.CommandText = "SELECT ID,Name from
Vehicles";
oleDbDataAdapter1.Fill(driverVehicleComb);

So the first time I run the command above (I left out a few lines but
it works) it runs very quickly. But the second
time I run the same method, on the DataAdapter fill method, it takes
more than 300 milliseconds to fill the table again.

Anybody know why?

I solved the problem by setting the driverVehicleComb to a new
DataTable instead of clearing it, but I was just wondering why this
happens....