About translog lastCommitted TranslogFileGeneration

The method commit() of Translog,which set lastCommittedTranslogFileGeneration to current.getGeneration(),why not set to current.getGeneration() -1 ?
Because the field current was made to new TranslogWriter by createWriter(current.getGeneration() + 1) in method prepareCommit(),so that i think it should be setted to current.getGeneration() -1.