Zen is NOT using CGNAT
Ok, I wasn't too sure.
The most likely explanation if poster is not seeing this variation when they do a ping string from a PC, is the DoS protection on the Fritzbox When using a test connection is may not have been up long enough for the DoS protection to kick in.
That might explain the odd issue I had after 10 days where I would see packet loss ever X hours.
Its a shame the FRITZ!Box doesn't let the user add an IP / domain or a trusted list to bypass that feature.