Default [Q] Why so much io with f2fs?

I often use "iostat" command to find out io usage on my device.
When all my filesystem are ext4 I never found the read data amount over 5GB in one day ,but after I formatted fs into f2fs the read amount is always over 50GB in 24hours (at least iostat told me so)
My daliy use ,AGNi kernel has turned "background_gc" off. But average read speed is still over 1500kb/s and this includes time when I'm not using my phone. How could this be ?