lxtx collects each process performance data from files in /proc/[PID]/.
This directory also has fdinfo, and lxtx uses this information to collect differential position of opened file.
This value might be usually how much sequential read or write is going on, even though this differential value represents just change of file position per unit time.
Below is example during file upload by sftp. Meanwhile, network traffic is 2.5MBPS to be read, and the differential file position is 2.39MBPS.