In the "process" tab, the I/O read column contains the write I/O that the
process is doing (and same for the I/O write column). Total I/O read/write also
have the same issue. iotop shows the correct value for read/write, so it seems
the problem is specific to gnome-system-monitor. I've also checked in french
and english, in case it was a case of bad translation, but the behaviour is the
same.

Apologies for the screenshot in french, header are (from left to right) :

Process Name - pID - state - %CPU - start time - memory usage - owner -
total disk read - total disk write - current disk read - current disk write