FMS Arrivals - Top of Descent

Watching several Air Canada Airbuses (Airbi?) descend into Halifax, NS (CYHZ), I started to wonder why some of these aircraft, all FMS-driven on the same inbound route and FMS STAR, all seemed to start descent at vastly different points and at vastly different rates of descent. I was comparing primarily A320 with A320, though the similarities with the A319 and A321 came into play as well. Some start down early and descend slowly, while others hang up forever and then dump. I took the opportunity that presented itself one midnight shift to ask a pilot when he was the only one on that particular frequency (we work many from one position through the night).

He told me that the FMS will generate a profile based largely on weight, which will affect TOD and descent rate. Makes sense. He also said that when the FMS generates its plan, the pilots can push taht back or forward to stay up high out of low level turbulence, icing, etc, a little longer, or start down earlier for traffic or whatever. Then, they mentioned the minor differences between the A319, A320 and A321. Then another voice, obviously the other guy at the helm, said, "And besides, we're in an A321 and they can be a real prick to slow down, so we started down early." A good point, though put a little bluntly.