Confused by disk usage

Hi forum
I’m using FreePBX and Asterisk
The System Status gui page reports disk usage that is ~ significantly different from using “df” at the cli.
Variances between partitions of 5 - 10%, same for / and /boot
Is this normal ?

most of the servers I looked at show a bit less on the GUI and with df -h. but then why does it matter?

It matters because my trimmed down freepbx server typically runs with / usage of 70-80%.
So a variance between actual usage and the gui displayed usage of 5-10% is significant.
Its also very confusing to receive those hourly emails from FreePBX that disk usage exceeds 75%, which could be wrong.
When time permits, I’ll try and find where the gui gets its incorrect data, and whether its fixable.