Just for context I have a factory unlocked S20 FE (SM-G780G) which is in the upper half of that list and I only got the download to my phone early Friday morning 3 days ago. A51 is very near the bottom. As said by
@Glenntech if you got your phone through a network carrier it will take longer to receive the update because Sky will then have to tinker with the update given to them by Samsung for your device model before then sending it to your phone for updating.
Are you based in the UK with a phone bought from Sky in the UK? Generally countries like South Korea, Germany etc will receive updates before the UK. Lots of factors to consider. Android 13/One UI 5 isn't radically different I wouldn't get hyped up as you may be underwhelmed when you do get it. Anyway good luck to getting it sooner.
If you want it as soon as possible and have a PC (works with Mac's too) you can get Samsung's official 'Smart Switch' app installed on your PC or Mac and connect your phone to your computer. Be sure you have 'Samsung Smart Switch Mobile' downloaded on your phone from the Google Play Store. If for some reason you can't download from the Play Store you should be able to find Smart Switch by going to Settings > Accounts and backup > Smart Switch and you can get software updates via this method faster. If I remember there is a triple dot menu icon at the top right and you can check for updates that way.
https://www.samsung.com/ph/support/mobile-devices/how-to-update-software-using-smart-switch/
