Thanks. Might've already been set that way but I did it again.
If the behaviour doesn't happen every single time, I suspect there is a race condition in the operating system's software, possibly around the use of multiple cores in parallel when initialising networking devices. The service order should be adhered to but I guess it's considered efficient to start up different devices just in case and the first one to finish gets precedence for a while. I haven't found a way to disable WiFi completely on Apple devices other than removing the WiFi hardware card itself. I gather such Apple devices don't really have an equivalent of a registry - but instead use .plist preference files. I have no idea if those are user modifiable to disable WiFi - probably not worth risking with those.