trusted_networks why /16 network

[Dhawal Doshy]

My organization is allocated a /19 network by apnic. My trusted mail 
servers (mx, smtp and delivery) all fall under a single /24 that i could 
set manually using the trusted_network setting but i'd prefer it to be 
automated out-of-the-box.

From Mail::SpamAssassin::Conf
if the 'from' IP address is on the same /16 network as the top Received 
line's 'by' host, it's trusted

Why does SA default to a /16 network and why not a /24 to be safer? OR 
am i missing something?

- dhawal