I have the Verizon Quantum router on 300mb FIOS internet and Verizon speed test gives me:
255mb down and 325mb up
304mb down and 360mb up.
I also thought device speed was speed taking computer limitations into account and Router speed was Router to internet without computer limitations.
But I get faster Device speed so that doesn't make sense.
NOT AN ANSWER, but:
I would try to replicate the results with various devices and various times of the day, if possible. If this happens only once, i'd chalk it up to a glitch.
You are correct - you cannot possibly have a faster connection that your router when you are, indeed, going through the router.
On a 300/300 speed you should be getting about the same speeds or a little over from your router. Not less. 320/340 etc. it’s not a magical stopgap at just 300/300 sometimes it can be less.
now at less than a gigabyte it is possible to obtain router speeds and device speeds at near or over the 300/300 speed. Depending on the device like iPhones you could get a faster device speed. But like the other poster said it just could be a fluke.
years ago with cable internet my iPhone had the same speed as my then modem/router it always amazed me.