Monitoring->Graphs displays different data

[earth03]

Hello,
I usually check DB resources on Monitoring -> Graphs.
I especially check the following item.

-item information------------------------------------------------------------------------
Name: check nfs free
Type: Zabbix agent
key: vfs.fs.size[/media/backup,free]
New flexible interval: Interval (in sec) 50 Period 1-7,00:00-24:00
---------------------------------------------------------------------------------------------

There was difference between Zoom:1h and Zoom:7d about [avg] last data though I check the same data at 9:50 am.
ex)
1h ⇒ 41.5%
7d ⇒ 34.6%

I checked server data,and 1h's one seemed correct,it's the same data to server and other monitoring app.

And then,there were no differences after 10:00 am.
ex)
1h ⇒ 41.5%
7d ⇒ 41.5%

I expected there was an only view difference between Zoom:7d and Zoom:1h.I can say it's not happened only one time.

I thought Zabbix server calculated trends for last 60 minutes at start of every hour.When data on graph taken from trends - last value was also from trends only.
But it doesn't seem that Zoom:1h acts like that. it's always updating its data.
It happens both ver 2.2.6 and 1.8.5.

Could anyone who knows this let me know why this difference happens,please?