Since expected losses can be reasonably estimated based on past experience, the provisions to cover the losses are made from the current earnings of the bank. Provisions can be thought of as an expense from the income of a bank to mark a non-performing loan to its economic value in the books of banks.
Sometimes, the actual realisation from a NPA could be higher than its marked down value, in which case banks write back the excess provision as profits in the accounting year in which the recovery takes place.