As mentionned by Justin, this is not a bug. I don't really find any documentation about this but the source code of recent version of procps seems to be quite sufficient (unless i'm wrong).
In minimal.c: [...] static void print_proc(void){ [...] case 'f': printf( "%8s %5d %5d %s %s %s %s", do_user(), P_pid, P_ppid, do_cpu(0), do_stime(), P_tty_text, do_time(P_utime+P_stime) ); [...] -- Guillaume Delacour
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