When you write a resource block for an existing resource prior to import, it is easy to not quite match the existing state as Terraform sees it.
My normal workflow is:
Write resource block
Consider changes in plan, edit resource block, re-plan, repeat as needed
Also I have found some cases where effectively no-op attribute changes show up on the first plan after an import - often null changing to empty list or things like that. I imagine that this is technically a (mild) bug in the provider, which should have import implementations that correctly set these in the first place.
So, there are multiple reasons this could happen.
To help further, you’d have to show us the actual resource definition and planned changes.