This is due to the rounding of 3 decimal places under the order details page. Rest assured, the full dollar amount you've entered has been used to purchase your security.
Here's an example of what you might see. With the quantity showing as 0.033 on the order details page, it may seem that the purchase value is only $98.29. However, the transaction value is indeed $100.
This is due to the order quantity being rounded to 3 decimal places. Please click on order history instead to see your order quantity reflected in 9 decimal places.