Just did 2 consecutive runs to further test your theory. First run, we killed Corpus head last and got the Systems as you predicted. Second run, we wanted to kill Infested last but someone, i don't know who, was dumb enough to kill it before we killed the Corpus head. This time, we got the Chassis.
TL;DR 2x Corpus head last: 1x Systems, 1x Chassis
I can confirm your theory is wrong. Maybe one head is more likely to reward a different part, but even that seems unlikey (and also way harder to verify).
It is completely randome on PC I have done over 45 runs and I have tried them in every way and it's randomized, so keep grinding guys and gals cause theres no surfire way to get any one paticular piece.
Okay so I checked, and the results aren't what I expected. I killed the grinner head last 3 times, got the chassis every time I did that. Killed the corpus head last, got the systems. Killed the ancient head last, got the helmet. I still cant fully confirm, but if your having trouble getting nekros, give it a try. At the very least it couldn't hurt.