du reports free space by scanning reachable inodes and calculating their size.

If a file is created, opened, and later deleted while it is still open, the program that opened it will continue to have access to the file, and writes and reads can be performed to that memory location on disk. du does not account for such open files whereas df does.

A simple restart should flush /tmp and solve your problem.

An alternative would be to scan open files and restart or kill the process that did not release the deleted files.