Yes, the actual order the channels are used is hard-set in the firmware of the controller, and can vary for different reasons. For example, a system in our area is set as follows:
Channel 9 - regular channel
Channel 8 - regular channel
Channel 7 - regular channel
Channel 4 - regular channel
Channel 5 - phone patch capable
Channel 6 - phone patch capable
Channel 1 - encryption capable
Channel 2 - encryption capable
Channel 3 - encryption capable
As you can see, the patch & encryption channels are further down the line. This is done to try & keep them free for patch & encryption requests. The order resets after a certain amount of time, so if the system isn't busy, channel 9 will be picked nearly every time. Even on a semi-busy system, the last 3 channels would only be rarely used for a regular voice call.
To muddy up the waters a bit more, a technician can go into the controller settings and 'protect' one or more channels, which forces it to the bottom of the list. This is sometimes done if a channel is receiving intermittant interference which is difficult to isolate.
Todd