Is there any logic behind STM bus numbers?

Radio-Canada has an audio excerpt asking whether there’s any logic to the STM’s bus numbering system and the expert says no – there are groupings like 400-numbers for express routes, but the rest, not so much.

This streetcar map from 1941 shows a few familiar routes – the 15, the 17 and the 55 are there – but most of the rest have since changed. The present system has evolved gradually over time, and nobody recorded why specific numbers were chosen. Oddly enough, Wikipedia has a complete list of routes including obsolete ones.