I did now a measurement and speedtest-cli measured 256 Mb/s down, 196 Mb/s up. Ookla running from FF measured 265 Mb/s down, 212 Mb/s up.
To be honest with you this is the first time these two measurements are this close to each other. I am no expert, but based on my experience sometimes you have to measure multiple times, as the maybe the selected server for measurement is under load. These speedtests select the servers based on their temporary ping, so I could measure completely different results using the same server, just starting Ookla again and again.
I would also check if your Zorin is not doing something in the back. Again, I am no expert just sharing some experience.
EDIT:
Maybe you should check this thread too.