iotop showing 1.5 MB/s of disk write, but all programs have 0.00 B/s

I don't understand iotop output: it shows ~1.5 MB/s of disk write (top right), but all programs have 0.00 B/s. Why? The video was taken as I was deleting the content of a folder with a few millions of files [using](https://unix.stackexchange.com/a/79656/16704) perl -e 'for(){((stat)<(unlink))}', on Kubuntu 14.04.3 LTS x64. iotop was launched using sudo iotop.