When CiviCRM asks a payment processor like PayPal to get it to process a payment, some of them tell CiviCRM how much they have charged in fees on the transaction.
(NB Not all payment processors provide this information at the time the transaction is made, and perhaps not all payment processor extensions for CiviCRM record the information even if it is available.)
I would strongly discourage modifying this fee information since you are likely to make it so that CiviCRM has different fees and thus different amounts deposited in your accounts than reality.
If you use a payment processor that does not return the fees, then it might make sense for the payment processor extension developer to calculate the fees and record them according to the fee schedule in effect at a certain time.
The problem with this, however, is that the schedule will change, and then there will be a period where the payment processor extension might not be using the correct calculation. Even if the plugin is updated in a timely way, not all users of the extension are likely to update it in time. A fee of zero is obviously wrong, but an automatically recorded fee 2.13 gives the semblance of accuracy. Probably best for any plugins that do record fees based on their own calculations to post a warning about this.
