Can't Mix Smoke Models....really?

[tinyelvis77@xxxxxxxxxxxx]

I have a Caddx NX8 panel, and am finally adding an additional two wire
smoke that I had originally planned while installing the system.

I bought a Sentrol (521BXT) to add and already have installed a System
Sensor (2WT-B).

As I'm wiring it up to the panel, I see a note indicating that both are
compatible with the system; but there is a big note not to mix
models.....Oops.

Why is it that they need to be identical?