DC-9 being delivered after MD-81 S3A

I have ordered some MD-81 as soon as they became available but also wanted to get some additional DC-9 while I wait for the former to start getting delivered. The system automatically put the additional DC-9 orders in the same wait list as the MD-81, which means I’ll have to wait two game years for them to arrive. In the Game they are two separate types, so why aren’t they differentiated in the delivery queue too?