"modern 9-pin serial ports"
Another fine mess.
D25 is numbered 1 to 13 along the long side and then 14 to 25 along the short side. D9s are likewise 1 to 5 & 4 to 9. PCB headers are odd numbers on one side and even on the other. The PCB header for RS232 was laid out so that pin 1 connected to pin 1 of the D, pin 2 to pin 14 & so on so you could use a ribbon cable and IDC connectors to connect the one to the other.
When someone designed the PCB for the D9 they obviously got a pin-out list but not a diagram for the D25, drew up a simplified list for 9 pins and used the same number list for the D and the headers. Now you can't make up a simple ribbon/IDC - put an IDC on the header, convert to a rat's nest at the D9 and solder it in. Whoever did it must have been a Very Senior Designer; it must have been spotted instantly when it went to the workshop to be made up but nobody sent feedback to get it corrected.