I am seeing a pattern, but I'm not sure its hard and fast.
Where brigades have specialist units (particularly hazmat and BWC) and that resource is required at an incident, then that code is sent. For "general" incidents then a different code may be sent.
Some examples, Burnside 2928 for a hazmat response, but maybe 2932 or 2924 for other incidents - I am still monitoring the paging site to confirm. The suffix, as mentioned, was determined by what was the mainstream appliance for that brigade when the database was set up. Or does Burnside bet paged 2928 regardless and then select appropriate resource by the info given in the page? Again, 28 because they were a Hazmat brigade.
Happy Valley did have a 32 appliance (later changed to pumper designation) but now with a 24P and 34. Code 8032 is still used for all MFS paged turnouts. HV do not have RCR, Haz or BWC.
Very few incidents would specifically require a 14 response, so no code is used to reflect that - unless there are brigades around that only had a 14 unit when setting the database - but none come to mind. 14 is almost always a backup to the primary responding appliance.
Stirling get paged with 9019 regardless of incident - is that because they were a RCR brigade at the time the database was set up (and still are), and/or because they carry RCR equipment on their primary appliance (Pumper)?
Of course I may have this all wrong, but I haven't been studying the paging site THAT closely, regardless, these are my interpretations and opinions only, not based on any written fact or document.