You are not using a serial connection, you are using a USB CDC/ACM connection, and that is a very very different thing.
With a serial connection when you send data it gets sent regardless of whether there is anything listening for it - the TX IO pin toggles high and low regardless.
However, with a CDC/ACM connection it's only possible to send data when there is something actively listening at the remote end - that is, when the port /dev/ttyACM0 has been opened by an application.
Until that port has been opened any data sent will remain in internal buffers. These buffers are 64 bytes each in size (that's the size of a CDC/ACM packet), and there are (I would guess) 10 of them on the Teensy3. After these buffers are full, because there is nothing receiving the data, there can never be any more room made for more data, so instead of blocking like a serial connection does it just discards the data.
So you have filled the buffers with 640 bytes of data and thrown away the rest. That data just sits there.
Then eventually you open the /dev/ttyACM0 port and the Teensy3 is now able to send CDC/ACM packets through the USB. And it has 10 of those packets sat there waiting to go out, so it sends them. So you receive the 640 bytes of data that you had buffered up.
Your mistake is basically not waiting for the port to be opened before sending data to the remote end. You can test the state of the port (kind of, but that's a whole other subject) by examining the Serial object:
if (Serial) {
// ... the port is open
}
In general you can usually just wait at the start of your program before starting anything else with a simple while:
while (!Serial); // Wait for the port to be opened