The answer (I was there, consider me an authority ...) is that the
PIDs needed to be somewhat unpredictable.  This is because AIX was
originally designed with a number of C2 and B1 features, and one of those
is the notion of covert channel analysis.  Sequential PIDs are a covert
channel (assuming the system has one PID namespace ...) since the value
of the "next" PID is shared by all currently running processes.  So if I
want to sneak some of my classified data out to your co-operating
non-classified program, I can do it by carefully controlling the value
of the "next" PID.

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