Yes, qdisc every packet goes through the qdisc. But here's a typical sequence of events in the situation I'm talking about:
- packet 1 enters qdisc
- packet 1 leaves qdisc
- packet 2 enters qdisc
- packet 2 leaves qdisc
There isn't much the qdisc can do here, prioritization-wise -- even if packet 2 turns out to be high priority, it can't "take back" packet 1 and send packet 2 first. Packet 1 is already gone.
In your tests you used bandwidth throttling, which moved the chokepoint so that it fell in between the qdisc buffer and your device buffer. You told the qdisc to hold onto packets and only let them dribble out at a fixed rate, and chose that rate so that it would be slower than the rate the device buffer drained. So the device buffer never filled up, and the qdisc actually had multiple packets visible at once and was able to reorder them.