The 0.8m accuracy for civilians is under ideal conditions and only horizontal position. Vertical positioning based on GPS is very bad so you're not going to see sub meter accuracy, maybe num even sub 5 meter accuracy. Again, like Rich said, baro can be affected by various things. If the weather changes even during a short flight, you'll see variations in pressure that can lead to errors up to what you're seeing. I looked at lots of barometers, all in the same location over a 10 minute span and while many tracked similarly, some were off by quite a bit over that short time.
Our sensors on these things are far from perfect. I bet if you log what your altitude offset is at landing for the next 10 flights, just on your own vehicle, you'll end up with a range of values that are good and bad. If they're consistently bad, then contact 3DR. If not, then it's your standard GPS and baro errors from low cost sensors.