It's pretty common for these transactions to be a few cents off due to rounding.
As an example, suppose you contribute $500 for a fund that has a per-share price of $56.03 on the date of purchase. They'll calculate the number of shares like so: $500.00 / $56.03 = 8.92379... They'll round it off to the nearest thousandth of a share, so they'll credit your account with 8.924 shares. 8.924 * $56.03 = $500.01, so you actually made a penny off the deal.
If the share price was instead $56.02, the calculation would go $500.00 / $56.02 = 8.92538... They'll round it off to 8.925 shares purchased. 8.925 * $56.02 = $499.98, so you lost two cents by rounding in this case.
Sometimes the rounding will be a few cents in your favor, sometimes it will go the other way. In the long run it will even out to about zero. Don't worry too much about it.