Just for discussion's sake, I've had this question for the longest time too, and discovered the answer by accident.
I was having some trouble with the ethernet NIC adapter on my laptop, and decided to update the drivers
to the latest ones.
Before I updated it, it showed the sent and received data
After I updated it, it showed the data as packets.
Just to be sure it wasn't a fluke, I performed a rollback to the previous drivers,
and it showed the data as bytes
This has led me to the conclusion that the network card's driver
plays a role in the type of data that it displays in that box. (The drivers would report the data to windows as packets or bytes, which is then shown on your status box)