Only reason I can think of is the speed difference, .65M for the 146, .68M for the Avro. This is one of several cases where combining the fleet groups and overlooking the very minor differences would be of great benefit to AWS as a whole. Another one would be DC-9/MD-80/MD-90/717. In the case of the DC-9 to 717 group, the speed is the same, the frame is the same, the wing is the same, and all share a common pilot type rating (http://fsims.faa.gov/PICDetail.aspx?docId=FSB%20DC-9%20R7
). DC-10/MD-11 falls into that category as well. All 737 series share a type rating with the only real difference between groups being speed increases (.72 -> .74 -> .78).
With how severely airlines are penalized for operating more than 3 fleet types, and with no relief in sight for this problem despite many cries for change over the years, simple combinations like this would make transitioning fleets over the years much less of a hassle and greatly improve game play in long worlds.