Question about NTP clock accuracy with RPI3

[nt74]

I have installed rocky-pi OS on a RPI3 and dwm desktop. I have also installed all necessary packages to be able to run the OnAirScreen app. The NTP clock is syncronised via chrony daemon and I am using our in-house NTP-clocks as well as ntp.org pool as backup. I have also installed a fake-hwclock (for RTC). No matter what I try to configure when I compare to our PTP Grand Master clock (Clock readout) and "Atomic clock" phone app, the OnAirScreen clock is a bit late, approximate 0,5 second late.

Is there any adjustment and/or tweaks I can do?