Most stock data firms make you agree that the information displayed may be inaccurate. If you want guaranteed accuracy, they make you pay through the nose.
Even for professional investors it frequently happens that you only know your exact PnL after markets close. With all automation there are still manual processes and software errors so manual trade reconciliation isn't that uncommon for high trading volumes.
But professional investors also have their own bookkeeping and deeper pockets to pay for losses.