When entering a PP on a pledge, in Batch Detail, for an amount that would overpay a bookable pledge, and the "Allow Overpayment of Bookable Pledges?" system preference is set to 'N', then you get a popup giving you the option to either manually change the payment amount or to split the payment into a payment that fulfills the pledge and a gift. If you choose to split the gift, the resulting PP and GF may not add up to the amount that was originally entered, and the PP itself less than the amount that is required to fulfill the pledge.
This may occur if there is another unposted batch item in the batch or another batch that has a PP for this pledge. The amount on the other batch item is taken into account when calculating the amount that needs to be paid to fulfill the pledge and affects how the amounts are split.
Resolution is to either delete the other batch item that is throwing off the current batch item then reenter the current batch item, or post the other batch item if it's supposed to be posted and reenter the current batch item.