Its not that VE8AT is open... "open" means the band is presently acvite and long distance communications are possible.
If you are not familiar with the ionosphere, I found this primer that explains it all pretty good. http://www.ae4rv.com/tn/propflash.htm
As I said, its not spectacular that you are hearing a beacon on 20, 17 and 15 meters. Those bands are common for long distance signals.
**NOTE** VE8AT is roughly the same distance from the state of SC as is northern California. We live on a round planet - not a flat map.
At what power level are you hearing the beacon?
NCDXF Beacons transmit the callsign and the first dash at 100 watts. The remaining dashes are sent at 10 watts, 1 watt and 100 milliwatts.
At the 100 and 10 watt levels hearing VE8AT, with moderate band conditions, is easy. Hearing the 1 watt and 1/10th of a watt power levels is much more difficult.
As the solar cycle starts to swing back up over the coming years you may well hear them at the 2 lowest power level. If your can hear the 12 and 10 meter VE8AT beacon now it means there is a brief opening. Again, as this solar cycle is on the upswing you will hear those freq's more and more and for longer periods.
Myself and many others do QRP operations and can easily communicate around the world with 5 watts or less, thanks to the ionosphere's magic and skill on the part of the operator.
I suggest you Google for HF propagation and get a cup of coffee and a note pad. It's not a simple subject with one answer. There are many factors that come into play. No one can tell you exactly which way the signal is bouncing to you or how many times it bounced to reach you. Ionosondes and Over The Horizon Radars (OTHR) can measure these things but its not something the amateur operator needs to worry about nor try to figure out.
Read up on space weather, solar storms, sunspots, greyline (grayline) propagation, the F layers, ionosondes etc. Above 50 MHz different conditions come into play.