Quoted By:
The reason for the confusion is because the Royal Train arrangements were all part of Operation London Bridge, which itself contains Operation Unicorn; so someone, either at Network Rail or the Royal Household, made the last-second decision to fly Brenda from Scotland to London, instead of following the plan that's been current and planned and quietly rehearsed by anyone who's anyone since 2017.
The reasons are all speculation at the moment, of course; maybe it's simply because she died so quickly and peacefully, rather than having a long illness that could have quietly kept a few things on standby. Or maybe it's just someone shitting themselves and making the wrong call, and now we're having to live with the results of that call. Who knows?