No rhyme or reason...
Upgrades or “offers” come to you for several reasons:
Inventory: If RCCL has LOTS of rooms left and they are trying to just move them, they’ll call out to make offers. That fills the more expensive rooms and frees up the better priced inventory for an easier sell.
The other reason is that there is some behind-the-scenes issue that, for some reason, they need YOUR room…. So they’ll call and entice you to give up your room for something better.
It’s just a luck thing if you are offered an upgrade.
It can definitely happen, but don't plan on it.
Hope that’s helpful.