ted:
That's NRZI signaling (link in post #35) with a signalling period of 83.3 nanoseconds (post #21) from a USB full speed (12 MHz signalling rate) bus.
The confusion rises (at least) from not being clear about the distinction between signalling on the TTL serial bus (at the baud rate specified in Serial.begin()) and the signalling on the USB bus. They are not the same physical bus and do not run at the same rate. For a USB to serial converter the USB side runs at 12 MHz regardless of the TTL serial rate.